











^■:%'X' ^0^ 



l:i 



^^ n. '/,"*«***'. ^ <*' 






o5? 




N*^ 



^N^ 






^<^ .# _^:^_fc %,# 










x^" ^-^ 



.^^^^ 









%.# 






^.>'^^' 



..^^^ -^ V^^.^ £ ^^ '% 



^ ^ ^ ^ /^ 









.^^ °^ 



.' ^^^ '?'• 



%^"''^ A-*- ,.„/^0^"''"-^^*' 









iil 






:^ 0^ 















.''^^^\/^ ... -o'^n^.\r^ . „ , V"7!!-^>^ 





















/ % \^ 



'^0^ 






^^M£^'%<^^ /«/^> %<^^ :^Mk\''-^^^' 



.^ °- 




^'%'-mwj/% 






,^°^ 














l'\^^^ 




"^^r..^^ 



^^V 






s 







X' 




'%<^'' 



c ^ 







^^^0^ 



.* .^(^^ 



ff "^ 



















%,^^ - 







'^=\,^^ 

c,*^ ^ 

«? ^ 






0^ 



* 80 



.<6 Q. 



,v ^ 




r ^ 



0^ :^iim\ '^^o^' r^<^'^^ ^^^^ 



^ ^ 



' ^ ^ 
* -?j^ ^ 







cS ^^ 



iifni 



cS ^, 










^^0^ 



N^ .^^' 



<^°^ \ 








^V ^ 


















- .o.^'^ 



^^^^- 










*^ °- 




Q-, "yT. s 




laiiiln-rlaiii L'HO VVrisluu^-tiui tiU'ciuJ^.j 



ELEMENTS 

OP 

NATURAL PHILOSOPHY; 



AND COXTAIXING 



DESCRIPTIONS OF INSTRUMENTS, WITH DIRECTIONS FOR USING. 



DESIGNED FOR THE USE OP 



SCHOOLS AND ACADEMIES. 



A. W. SPRAGUE, A.M. 



WITH TWO HUNDRED AND EIGHTY ENGEAVINGS, 

BOSTON: 
PHILLIPS, SAMPSON & COMPANY, 

13 WINTER 8TBEET. 

>1856. 



Entered according to Act of Congress, In the year 1856, by 

PHILLIPS, SAMPSON & CO., 

In the Clerk's Office of the District Court of the District of Massachusetts. 



i Stereotyped br 

- ) ^^ HOBART b R0BBIN8, 

I ^ New England Tjpc and Sttrcotjpo Foundery, 

BOSTON. 



J^l 



("To 
REV. EDWARD HITCHCOCK, D.D., LL.D. 

WHOSE DISTINGUISnED LABORS IN THE FIELD OF PRACTICAL SCIENCE, AND "WHOSE PURE AND LOFTY, 
TET MILD AND GENIAL BEARING, BETOKEN THE CHRISTIAN AND PHILOSOPHEBj 

BT HIS FORMER PUPIL, 

THE AUTHOR. 



PEEFACE. 



The principles of Natural Science are best comprehended 
by visible illustrations. But few minds obtain a clear under- 
standing of the operation of Nature's laws from mere written 
or oral descriptions ; the eye must see the modus operandi 
before the mind can gain a full and just comprehension of the 
principle ; an interest must be awakened by an ocular demon- 
stration before the attention can, in most cases, be sufficiently 
secured to fix the idea. Hence the importance which has come 
to be attached within a few years to the use of philosophical 
instruments for scientific illustrations, and the rapid and gen- 
eral introduction of these into the various seminaries of the 
country. 

The skill requisite for the successful and economical use of 
a philosophical apparatus is far greater than is often supposed. 
For this reason many teachers and lecturers, thoroughly con- 
versant with the theories of Natural Philosophy, fail sadly in 
their attempts at a practical illustration of its principles by 
means of instruments. No one familiar with the theories merely 
of steam or electro-magnetism, as learned from a general study 
of these subjects, could reasonably expect, without some specific 
1* 



VI PREFACE. 

practical directions, to run a locomotive, or operate successfully 
a telegraph ; in either case such an attempt would most likely 
result in a failure, if not in the positive injury or destruction 
of the machine. As well, however, may the teacher, conversant 
only with general science, hope to operate successfully the Air- 
Pump, the Oxhydi'ogen Microscope, or the various other instru- 
ments of a philosophical apparatus, without the aid from explicit 
practical instructions. 

From long personal observation, we are satisfied that a want 
of the requisite practical directions renders useless a very large 
portion of the apparatus for scientific illustrations purchased 
and deposited in the various institutions of our country ; thus 
depriving the pupils in those institutions of a large portion of 
that practical instruction which their wants demand. 

Text-books of Natural Philosophy seldom contain any really 
serviceable directions for the inexperienced manipulator. In 
prescribing the more obvious rules, they too often pass over 
the real difficulties in philosophical experimenting. Nor is this 
a matter for wonder, since such works are so often prepared by 
those unacquainted with the structure and practical operation of 
the instruments described by them. 

From an experience of more than four years in one of the 
most extensive philosophical instrument manufactories, and 
after having been for several years engaged as a teacher and 
lecturer upon natural science, the author has become fully con- 
vinced of the want of an elementary treatise upon Natural 
Philosophy, which shall present in a concise yet intelligible 
manner the principles of this science, and at the same time 
describe the method of illustrating these principles, together 



PKEFACE. VII 

with the kind and proper use of the instruments requisite for 
such illustrations. 

With the view of meeting in some good degree such a 
want, at the solicitation of several teachers and friends of 
popular science, the author has been induced to prepare the 
following work. 

In treating of the subjects contained in this work, it has been 
our aim to present such a practical view of each as the wants 
of a practical age demand ; avoiding, however, those math- 
ematical formulas and those specific details, ill-suited to a 
work designed merely as a text-book for schools and seminaries 
of learning. 

Convinced by past experience that principles in philosophic 
science are much better comprehended and longer retained 
when accompanied with appropriate illustrations, we have 
endeavored to make such a selection of experiments as seemed 
best adapted for elucidating these principles and rendering 
them intelligible to the youthful mind. Accompanying these 
experiments are numerous cuts of the various instruments by 
which they may be successfully performed. Directions for the 
use of these, in connection with the experiments, have been 
given in the text, or notes appended, and also such liabilities 
and dangers as experience has suggested have been pointed out, 
and the proper cautions given. 

The subject of Astronomy, so generally introduced into text- 
books of Natural Philosophy, has been omitted in this work : 
this, in the opinion of the author, being better learned from a 
separate treatise, while Heat, which is often rejected as a subject 
exclusively within the province of Chemistry, has been retained, 
as properly belonging either to this or Natural Philosophy. 



VIII PREFACE. 

A concise and practical description of the construction and 
operation of the Magic Lantern, Oxhydrogen Microscope, and 
other articles of a philosophical apparatus, now extensively used 
for popular exhibitions of science, has been added. Such, we 
doubt not, will meet with favor from teachers and lecturers un- 
practised in the use of these machines. 

A cheaper class of instruments has not been described in the 
following pages, such being but little used, and, when used, the 
manner of operating them may be learned equally well from a 
more perfect apparatus. The directions given will apply in gen- 
eral to all styles of instruments, whether of American or 
European manufacture. 

A. W. S. 

Boston, January IM, 1856. 



TABLE or CONTENTS. 



PAGE 

PROPERTIES OF MATTER. 
Extension and Impenetrability de- 

"ned, 13 

Divisibility, Figure and Porosity, . 14 
Inertia of Matter (illustrations), . 15 
Attraction — two kinds defined, . 16 
Solid, Fluid and Gaseous Bodies, . 16 
Specific Properties of Matter, as 
Hardness, Elasticity, Flexibility, 
Malleability, Ductility, and Te- 
nacity (illustrated), .17 

Geavitatiox — tendency to draw 
all Terrestrial Bodies towards the 

Earth's Centre, 18 

Acts alike on all Bodies (illustrated), 19 

Gravitv, Centre — to find this, . 20 

Falls before point of support, ... 21 
Equilibrium — stable, unstable and 

neutral, 22 

Illustrations of Centre of Gravity, 24 

MOTION. 

Difierent kinds of Motion defined, . 25 

Forces retarding Motion, .... 25 

^Momentum, 26 

Laws of Falling Bodies, 26 

Atwood's Machine for showing these, 27 
Motion of Projectiles (illustrated), 28 
Central Motion; caused by the Cen- 
tripetal and Centrifugal Forces, 29 
The latter increases with the Dis- 
tance from Centre of Motion, . . 31 
Action and Reaction; shown by 

Collision Balls, 31 

Reflected Motion, 32 

Resultant Motion (illustrated), . . 32 

Problems, 34 

MECHANICAL POWERS. 

Machines, how composed, .... 36 

Lever; three kinds illustrated, . , 36 



PAGE 

Balance and Steelyard, 37 

Compound Lever, 39 

Wheel and Axle, 40 

Capstan, 41 

Pulleys, two kinds, 42 

Inclined Plane, 43 

Wedge, Screw, 45 

Machinery — of what composed — 

its advantages, 47 

Relation between the Power and 

Resistance in Machines, .... 48 

Fly-wheel, 49 

The Governor, 50 

Fusee in Watch-work, 51 

Pendulum described, 51 

Laws of its Vibration, 52 

Pendulum used for determining the 

Figure of the Earth, 53 

As a Measure of Time ; application 

to Clocks, 54 

Friction; Problems, 55 

HYDROSTATICS. 

Pressure of Liquids, 57 

Hydrostatic Press ; to ascertain 

Pressure on this, 58 

Equilibrium of Liquids, 60 

HjKlrostatic Bellows, 61 

Hydrostatic Paradox; illustration 

of Pressure of Liquids, 63 

Non-compressibility of Water shown, 65 

Specific Gravity (illustrated), . . 66 
To ascertain Specific Gravity of 

Solids, 67 

Of Liquids; Hydrometer, .... 68 

Floating Bodies; Swimming,. . . 69 

Hydraulics defined, 70 

Flow of Liquids; Resistance ofibred 

to Bodies moving through them, 71 
Water as a Motive Power; Water- 

Wheels; Barker's Mill, , . . , 72 



CONTENTS. 



PAGE 

Archimedes' Screw; Problems, . . 73 
Desckiptiox of Pneumatic Instru- 
ments, 75 

The Air-Puinp described, .... 77 
Theory of the Operation of the Air- 

Puuip, 79 

Directions for its Use, 80 

Condenser described, 82 

Siphon Vacuum-Grauge, 83 

Condensing-Gauge ; Guinea and 

Feather Tube, 84 

Screw Connections, Ac, 85 

PNEUMATICS. 

Atmospheric Air a Type of Fluids; 

its obvious Properties, 87 

Materiality of Air shown by its 

Visibility; Inertia, 88 

Resistance offered to Solids ; Guinea 

and Feather Experiment, ... 90 
Fall of Liquids in a Vacuum, ... 91 

Buoyancy of Air, 92 

Impenetrability of Air; Diving- 

Bell (illustrated), 93 

Air, its Weight and Pressure, . 94 
The rise of Liquids; how accounted 

for by the Ancients, 95 

To Weigh Air. The Hand-Glass for 

showing Atmospheric Pressure, . 96 
Cupping-Glass; Bladder-Glass, . . 97 
Magdeburgh Cups; Pressure upon 

these ascertained, 99 

Condensed Air ; its Effect on the 

Animal System, 100 

Porosity of Wood shown by Atmos- 
pheric Pressure, 102 

BoIt-IIead. Water sustained in an 

Inverted Jar, 103 

Height to which a Column of Mer- 
cury may be sustained. Weight 

of the Atmosphere, 104 

Absurdity of Suction, 105 

Upward Pressure of Air shown, . .106 
The Barometer and its Uses, . . . 107 
Boiling of Liquids under a Pressure, 110 

Atmospheric Telegraph, Ill 

Fluidity of Air shown, 112 

Elasticity of Air; Mariotte's Law, 113 
Hydrostatic Balloon ; Pneumatic 

Balloon, 114 

How Fishes Rise and Sink in Water 

(illustrated), 115 

Bacchus Experiment, 116 

Fountain in Vacuo, 117 

Elastic Force of Compressed Air; 

Artificial Fountain, 118 

Condensing-Chamber Charged ; Air-, 

Gun; Revolving-Jet, 119 

Pneumatic Paradox; Beer-Jar, . .120 
The Lifting-Pump Explained, . .121 



PAGE 

The Fire-Engine explained, ... 122 

The Siphon explained, 123 

Siphon Fountain; Hiero's Fountain, 124 

STEAM. 
Expansion and Elastic Force of 

Vapor, 126 

Marcct's ^team Globe; Eolopile, 127 

Atmospheric-Engine, 128 ; 

Watt's Improved Steam-Engine, . 130 

High-Pressure Steam-Engine, . . 132 

METEOROLOGY. 
Winds, how caused; Land and Sea 
Breezes, 135 

Whirlwinds and Hurricanes, . . 136 
Trade-Winds; Mists and Clouds, . 137 
Rain; Hail; Dew, how formed, . 139 

SOUND. 

How produced, 141 

Air a Conductor of Sound, .... 142 
Its Conducting Power varies, . . 143 
Solids Conduct Sound (illustra- 
tions), 144 

Liquids Conduct this; Velocity of 

Sound Progressive, 145 

Reflection of Sound; Echo, ... 146 
Ear of Dionysius; Speaking-Trum- 

pet, 147 

Musical Sounds; Pitch, 148 

Theory of Musical Sounds, . . . 149 

Wind Instruments, 150 

The Human Voice ; Ventriloquism, 151 

Problems, 153 

MAGNETISM. 

Natural and Artificial Magnets, . 154 
How arrange themselves when free 

to move, 154 

Theory of Magnetic Induction, . 156 

Magnetism of Soft Iron ; of Steel, 157 

Artificial Magnets, how made, . . 157 

Terrestrial Magnetism, 158 

JMagnetic Needle, 159 

Its Declination, . 100 

Its Dip, 161 

Introduction to Electricity, . . 165 
History of the Science; Theories, 

&c 165 

Electric Machine described, . . 167 

Leyden Jar and Dischargers, . . 170 

Electric Battery and Directing-Rod, 171 
Pith-Ball Electrometers; Hydrogen 

Generator, 172 

Gold-Leaf Electrometer, .... 173 

MECHANICAL ELECTRICITY. 
Electricity produced by Friction of 

Glass, 171 

Of Cloths; of Steam, 175 



PAGE 

Two kinds of Electricity, .... 176 

Theory of the Electric Machine, . 177 

Electric Attraction and Eepulsion, 178 
/Electric Repulsion; Sportsman and 

' Birds, 179 

! Dancing Images; Spider; Swing- 

j Bells, 180 

Electric Swan; Balance Electrome- 
ter, 182 

j Electriclnduction Theory, . . . 184 

Electrophorous, 185 

Theory of Leyden Jar, 186 

Induction shown by Double Jar, . 188 
j Electricity resides on the Surface 
! of a Glass Jar; Movable Coating 

Jar, 189 

Electric Illumiis^ation, Different 

Colors, 190 

Luminous Frame; Star, 191 

Luminous Tubes; Jar, 192 

Passage of Electricity through Rar- 
efied Air; Abbe Nollet's Globe, 193 
Aurora illustrated; Ealling Star, 194 
Combustion by Electricity. 
Universal Discharger ; Powder- 
Bomb, 195 

Electric Cannon; Manner of Filling, 197 
Inflammation of Ether by Electric 

Spark, 198 

Mechanical Effects of Elec- 
tricity, . . .• 198 

Decomposition by the Electric 

Spark, 200 

Electric Mortar, 201 

Electricity from Points, 201 

Agency in Evaporation shown, . 204 

Bil'ects on Animal Sj-stem, . . . 204 

Virtues as a Medical Agent, . . 205 

Electricity of the Atmosphere, 207 

Thunder-Clouds, 209 

Return Stroke, 210 

Lightning-Rods, 211 

Places of Safety from Lightning, . 213 

Aurora Borealis, 214 

GALVANIC ELECTRICITY. 

History of its Discovery, .... 216 

Manner Produced, 217 

Compound Battery, 218 

Galvanic and Mechanical Electric- 
ity; Difference, 219 

Galvanic Batteries, 222 

Decomposition by Galvanic Elec- 
tricity, 227 

Electro-Metallurgy, 231 

Heating Effects of Galvanic Elec- 
tricity, 233 

Effects upon the Animal System, . 236 



THERMO-ELECTRICITY. 
How Produced, and Direction of its 
Flow, 239 

Thermo-Electric Battery, .... 240 

ANIM5LL ELECTRICITY. 

Gymnotus, 241 

Theory of Muscular Action; Di- 
gestion, 243 

ELECTRO-MAGNETISM. 

Effects of a Flow of Electricity in 

causing Magnetism, 245 

Galvanometer, 246 

Astatic Needle, 247 

Reaction of a Magnet; its Revolu- 
tion, 248 

Polarity of a Magnetic Helix, . . 251 

Thermo-Electric Arch, 254 

re's Theory of Terrestrial 

Magnetism, 255 

' Electro-Magnets, how made, . . 256 

i Magnetizing Helices, 257 

I Electro-Magnetic Telegraph ; 

j Morse's, 260 

! Signal-Key, 265 

! House's Printing Telegraph, . . 266 
Fire-Alarm; Municipal Telegraph, 271 
j Machines to revolve by Electro- 
Magnetism, 272 

Attraction of Electric Currents, . 278 

Secondary Currents, 278 

j Shocking-Machines, 279 

' Magneto-Electricity, 283 

! 

I LIGHT. 

■ Two Theories ; Rate of its Progress; 
i Self-Luminous Bodies, .... 286 
Opaque, Transparent, Translucent 

Bodies, 287 

Course of Light; Shadow and Pe- 
numbra, 288 

Intensity of Light diminishes with 
the Distance from the Luminous 

Body, 289 

Photometer, 289 

Reflection of Light — Plane Mir- 
rors, 291 

Kaleidoscope, 294 

Curved Reflectors, 295 

Images from these, 296 

Convex Reflectors; Images from 

these, 297 

Refraction of Light, 298 

Limiting Angle of Refraction, . . 300 

Mirage; Lenses, 301 

Light on a Double-Convex Lens, 302 



XII 



CONTENTS. 



PAGE 

On Double-Concave Lenses, . . . 303 
Images formed by Lenses, . . . 304 
Decomposition of Light; the Prism, 305 

Achromatic Lenses, 307 

Rainbow, 308 

Polarization of Light, . •• . . . 310 
Calorific Eays; Chemical Action of 

Solar Light, 311 

Photograph, 312 

Daguerreotype; Hillotype, . . . 313 
The Eye, — its Structure, . . . . 315 
Images formed on the Retina, . , 316 
Power of adapting itself to Dis- 
tances, 317 

The Magnitude of Distant Objects, 

how determined, 318 

Near and Far Sightedness, ... 319 
Impressions remain on the Retina, 320 
Phantasmascope ; Spectral Colors, 321 
Inability to distinguish Colors, . 322 
Optical Instkuments, — the Micro- 
scope, 322 

Compound Microscope, 324 

Uses and Advantages of the Micro- 
scope, 326 

Reflecting Telescope, 327 

Refracting Telescope, 328 

Terrestrial Telescope or Spy-Glass, 330 
Galileo's Telescope, 332 

HEAT. 

Sources of Heat ; Expansion of 

Bodies by Heat, 333 

The Air Thermometer, 334 

The Mercurial Thermometer; Fill- 
ing of this, 335 



PAGK- 

Expansion of Solids; Pyrometer, 
Equilibrium of Heat ; Conductors 

of Heat; Metals good, .... 337' 
Liquids bad Conductors; how these 

become Heated, 339 

Radiation of Heat, 339 

Reflection of Heat, 340 

Transmission of Heat, 341 

Cold and its Causes, 341 

Cold produced by Evaporation, . 342 
Water Frozen by its own Evapora- 
tion in Vacuo, 343 

Freezing Mixtures, 345 

ADDITIONAL PHILOSOPHICAL IN- 
STRUMENTS. 

The Magic Lantern ; how Con- 
structed, 346 

Directions for the Use of the Magic 
Lantern, 348 

The Screen, 350 

The Phantasmagoria, 350 

Dissolving Views; Arrangement of 
Lanterns, 350 

Dissolving Stop-Cock, 351 

The Oxhydrogen Microscope; how 
arranged, 353 

Preparation of Oxygen Gas for the 
Drummond Light, 355 

Preparation of Oxygen Gas from 
Manganese, 357 

The Hydrogen Generator; to pre- 
pare this, 359 

To regulate the Flame of the Oxhy- 
drogen Jet, 300 

The Solar Microscope, 360 

The Benzole, or Water-Light, . . 361 



I 



NATURAL PHILOSOPHY. 



PROPERTIES OF MATTER. 

1. Natural Philosophy explains the laws -which regulate 
bodies, or matter in masses. This comprises mechanics, hydro- 
statics, pneumatics, electricity, magnetism, and optics. 

Matter possesses two essential properties, which are in- 
separable from the very idea of it ; these are extension and 
impenetrability. 

The extension^ or magnitude of matter, is expressed by the 
three dimensions, length, breadth, and thickness. 

Impenetrability is that property of matter whereby a body 
excludes from the space it occupies all other bodies. The 
proof of this, like extension, is too obvious to require demon- 
stration. Thus, when the point of a knife or an awl is thrust 
into a piece of wood, the particles of the wood are not pene- 
trated, but merely separated. So, when a spoon is placed in 
a cup filled to the brim with any liquid, the liquid flows 
over to make room for the spoon. Even the subtle gases, as 
air, are alike impenetrable ; for, if we force an inverted tum- 
bler into a basin of water, the presence of the air will exclude 
the water, nor can the tumbler be filled with this until the air 
is removed.* 

* This is beautifully illustrated by the experiments in § 78, Pneumatics. 

Define Natural Philosophy. What does it comprise ? The essential prop- 
erties of matter ? Define extension. Impenetrability. Illustrations of im- 
penetrability ? 

2 



14 PllOPERTIES OF MATTER. 

2. Besides extension and impenetrability, matter possesses 
other general properties, as divisibility, figure, porosity, inertia, 
and attraction. 

Divisibility of Matter. — Matter is supposed to consist of 
ultimate atoms ^ infinitely minute and indivisible, although we 
are unable to determine this by any perception of the senses. 
As an instance of extreme divisibility, we may adduce strych- 
nia^ an imperceptibly small portion of which will render bitter 
a whole pint of water, thus dividing and diffusing itself through- 
out every part of the liquid. So musk will continue for years 
to send off its particles, filling the room with a most intensely- 
penetrating odor, and yet suffer no perceptible loss of weight. 
Excellent illustrations of the divisibility of matter are furnished 
by some metals. Thus, gold may be hammered so thin that 
three hundred and sixty thousand leaves of it, piled one upon 
the other, will only equal the thickness of an inch ; and plat- 
inum may be drawn into wire so small that three millions of 
these wires, laid side by side, will scarcely extend over an inch 
in diameter. 

3. The figure of a body is its form or shape. All bodies 
have a determinate form, which is implied in the idea of exten- 
sion. This, however, is not confined to matter, since shadows 
and spectral illusions, which have no material existence, have 
figure. 

Porosity. — The empty spaces which intervene between the 
particles of bodies are termed pores. These vary greatly in 
different substances, and determine the o^ewsiV^/ of bodies. Thus, 
the pores of lead and gold being smaller^ the atoms of these 
metals approach each other more nearly, and they are therefore 
said to be more dense. 

That all bodies are porous, may be proved by the fact that 

Other general properties of matter ? Are the atoms of matter divisible ? 
Illustrations of the divisibility of matter ? What is meant by the figure of a 
body? Is figure confined to matter ? What are the pores in bodies? Why 
are lead and gold more dense than most other bodies ? 



PROPERTIES OF MATTER. 15 

they may be compressed and made to occupy less space, 
Avhich could not be done if their particles were already in 
contact. The porosity of many bodies may be also satisfac- 
torily shown by the fact of their admitting within them 
othe^ more subtile, without at the same time increasing 
their limits. Thus, into a cup filled with warm liquid a con- 
siderable quantity of sugar or salt may be thrown without 
causing the liquid to overflow, since the particles of the 
former diJOfuse themselves through the pores of the latter, and 
so occupy the spaces previously vacant. So, by means of the 
air-pump (§ 63), water, wood, etc., may be shown to have 
between their particles a large amount of space filled only with 
air and gases. ^ 

4. Inertia. — This is a property of all material bodies, by 
virtue of which they are incapable of moving themselves when 
at rest, or of stopping themselves when in motion. The motion 
of a body, therefore, supposes a moving power without itself The 
power which puts a body in motion, or which stops it when in 
motion, is termed /orce. 

The numerous examples of every-day life serve to eluci- 
date this principle of inertia in matter. A man standing up- 
right in a boat will fall backwards when the boat is suddenly 
pushed fi:om the shore, and forwards when it strikes the land 
again. In the former case, the whole body does not at once 
partake of the motion of the boat, but the feet, which rest 
upon it, doing this more rapidly than the upper portions, 
these latter fall behind, causing a fall of the body backwards ; 
so when the motion of the boat is suddenly checked, the feet 

* The atoms of liquids, as well as solids, are supposed to be globular. 
Thus, the particles of water are regarded as sustaining to each other positions 
similar to fine shot, and so allowing the smaller atoms of other bodies to enter 
and fill them. 

Porosity of matter, how proved ? Illustrations ? Define Inertia. What does 
the motion of a body suppose ? What is force ? Illustrations of inertia ? 
Cause of such results ? 



16 PROPERTIES OF MATTER. 

arc stopped, while the head and upper parts tend to proceed, 
causing these to fall forwards. The same law of inertia is 
seen when a person jumps from a train of rail cars in motion, 
his feet being stopped, while the head tends to move onward, 
causing it to be brought violently to the ground. 

5. Attraction is the tendency of matter, whether in atoms 
or masses, to be drawn together. When this exists between 
the molecules, or particles of a body, it is termed molecular 
attraction^ or cohesion ; * when between masses of matter, 
gravitation. 

6. Every body exists in one of three states, solid, fluid or 
gaseous, according to the force with which its particles are 
drawn together, or cohere. 

Solid bodies are such as have the position of their parti- 
cles Jixed in relation to each other, and require a force supe- 
rior to their own weight to change the form of the body ; of 
such are wood, stones and metals. 

Fluids have a cohesion between their particles, which holds 
these within certain limits, yet allows of their gliding easily 
among themselves. These, unlike solid bodies, have no inde- 
pendent form, but take the shape of the solid surfaces within 
which they are confined. Water and other liquids are examples 
of fluids. 

Gaseous bodies have a mutual repulsion between their 
particles superior to the force of cohesion, by virtue of which 

* When the particles of two separate bodies are brought suflBciently 

near, they adhere and become as one body. Thus, if 

Fig. 1. two perfectly polished glass plates have their surfaces 

^ cleanly wiped, and then be placed together, they will 

iWmiiii i iffliii Mmmi ffl^ adhere, by the cohesion between their particles, so firmly 

^7n.ui,T:jljmm|iu.uMn|||jjjjjji g^g ^q rcqulre a very great sliding force to move them. 

w Figure 1 shows the form of glass plates commonly used 

for this purpose. 

Define attraction. What is molecular attraction, or cohesion ? What 
gravitation ? The three states in which bodies exist ? What are solid bodies ? 
Examples? Fluids? Examples? Gaseous bodies ? Example? 



PROPERTIES OF MATTER. 17 " 

they tend to separate, and occupy a volume increasing in 
proportion a^ the external pressure upon them is removed. 
Of gaseous bodies atmospheric air is an example. 

7. Specific Properties of Matter. — Besides the general 
properties which have been described, matter contains other 
properties, found in a special degree in particular species of it. 
Such are called specific properties. Of these, hardness and 
elasticity, flexibility and brittleness, malleability, ductility and 
tenacity, are the most important. 

Hardness is the property of matter by which the particles 
of a body keep their relative positions, so as to resist any 
force which tends to change the form of the body. This is 
distinct from density, since the most dense bodies, like lead 
and gold, are often comparatively soft. ^ 

Elasticity is the property by virtue of which bodies, when 
compressed, tend to recover their former positions again. Thus, 
a steel spring, an ivory ball, India rubber, and atmospheric air, 
are examples of elastic bodies. Elasticity exists in bodies in 
different degrees, and when equal to the force which presses 
on the body, such a body is said to be perfectly elastic. Such 
are the bodies just mentioned. 

Flexibility and Brittleness. — When any body, as a rod 
of metal, readily yields or bends under a force applied to it, 
it is said to be flexible. If, however, instead of bending, the 
rod be readily broken, it is said to be brittle. Thus, a bar 
of steel, which has been heated, and then slowly cooled 
(annealed), is flexible, while the same, if suddenly cooled by 
plunging in cold water, is rendered brittle. 

Malleability is a property which metals possess in different 
degrees, whereby they allow of being hammered or rolled into 
thin sheets or leaves. Thus iron, zinc and gold (2), are 
highly malleable. 

What are specific properties of matter? The more important of these? 
Define hardness. Define elasticity. What is meant by perfect elasticity in 
a body ? When is a body said to be flexible ? And when brittle ? Define 
malleability. 



18 GRAVITATION. 

Ductility in metals enables tliem to be drawn out into wire. 
Platinum affords the most perfect example of a auctile metal 
(2), while iron and copper possess both malleability and duc- 
tility in a high degree. 

Tenacity in bodies causes their particles to adhere and 
resist a separation. Thus, iron and copper are highly tena- 
cious,* while tin and lead possess this quality in an inferior 
degree. 

GRAVITATION. 

8. If a stone or other body be dropped from the hand, and 
left free to itself, it falls until arrested in its course by some 
opposing obstacle, as the floor or ground. 

As matter is inert (4), and incapable in itself of motion, we 
ascribe its fall to the earth to a mysterious force termed the 
force of gravity. 

9. Gravity acts on all terrestrial objects in the direction of 
the eartKs centre. — From whatever portion of the earth's 
surface a body be let fall, its tendency is invariably towards the 
centre of the earth. f Thus, instead of bodies falling at all 
points on the earth's surface, in directions 'parallel to each 
other, they do this at angles varying with their distances. A 
body let fall at Boston will make nearly a right angle with one 
let fall at Cape Horn ; and two bodies let fall at the same time 
from Boston and Australia, will approach each other in nearly 
opposite directions. 

* An iron wire one sixteenth of an inch in diameter will support a 
weight of five hundred and forty pounds, while one of lead, of the same 
diameter, will support only twenty-seven pounds. The supei'ior tenacity 
of iron renders it highly serviceable in the construction of suspension 
bridges, and wherever great strength is requisite. 

t JNlany phenomena of matter appear to be contradictory to this law of grav- 
ity ; as the rising of balloons, the floating of clouds and various light bodies. 
Tliose, however, rise, and are sustained above the earth, by reason of the force 
of gravity ; a bulk of atmospheric air weighing more than these bodies sinks 
beneath, and thus forces them upwards. 



Tcfine ductility. Tenacity. Define gravitation. In what direction does 
g- .1 . itation act on all terrestrial objects ? Illustration ? 



CENTRE OF GRAVITY. 19 

10. Gravity acts alike on all bodies, and^ where 7io ob- 
stacles interjjose, these fall to the earth with equal velocities. 
— If a flock of cotton and a ball of lead be dropped together 
from an elevation, the latter will fall rapidly, while the former 
lingers in its descent. This difference is due to no superior force 
of gravity exerted on the lead, but to the fact of the air offering 
a greater resistance to the extended surface of the cotton than 
to an equal mass of compact matter in the lead. If these be 
allowed to fall in a receiver, from Avhich the air has been ex- 
hausted, they will both reach the earth at the same instant, as 
shown by experiment, § 75. 

11. The weight of a body is the force with which it gravi- 
tates towards the earth's centre, and this force is directly as the 
mass of matter contained in the body. 

The force of gravity diminishes in the same ratio as the square 
of the distance from the centre of the earth increases ; hence it 
follows that bodies elevated above the earth weigh less than at 
its surface. Thus, a body, weighing one thousand pounds at the 
level of the ocean, loses two pounds when elevated four miles 
above this ; and if carried from the earth to the distance of the 
moon, — two hundred and forty thousand miles, — and there 
acted on only by the earth, its weight would not exceed five 
ounces. 

For this reason bodies weigh more at the poles than at the 
equator, from the fact of the former being nearer the earth's 
centre than the latter. 

CENTRE OF GRAVITY. 

12. There is in every body a point, about which all the par- 
ticles composing the body balance each other. This point is 

Under what circumstances ■will light and heavy bodies fall to the earth 
in equal times, and why ? What is the weight of a body ? How does the force 
t)f gravity diminish as we go from the earth's centre ? Give an illustration. 
AVIiy do bodies weigh less at the equator than at the poles? Define the centre 
oi' gravity of a body. 



20 



CENTRE OF GRAVITY. 



Fig. 2. 



termed the centre of gravity of the body. Accordingly, if a 
body be supported at this point, all its parts will be in equilib- 
rium, whichever way it be turned. 

To find the Centre of Gravity. — When the particles com- 
posing the body are homogeneous (the same kind), and its form 
regular, the centre of gravity will be at the geometrical centre. 
Thus, in the triangular surface, 7 (Fig. 2), the centre of gravity 
may be determined by drawing lines from two of the angles, so 

as to bisect the oppo- 
site sides ; the point 
where these lines in- 
tersect each other will 
be the centre of gravity 
of the surface or body. 
If the body be a square 
or parallelogram, its 
centre of gravity will 
be the point where 
lines, joining the op- 
posite angular points, 
intercept, as shown in 5 and 6. The centre of gravity of these, 
as well as of homogeneous bodies, whose forms are irregular^ 
may be found by suspending them freely from one of their 
angular points, and marking the direction of a plumb line * 
let fall from the point of suspension, and then suspending from 
another angular point, and marking again the direction of the 
same line ; the centre of gravity will be at the point where 
these lines intersect. Thus, if the figures above be pierced with 
holes at two of their angles, and then suspended by these holes 

* This is sliown at 2, Fig. 4, and is simply a lead weight attached to a 
string. The plumb is used by artisans for ascertaining when the position of 
bodies is perpendicular. 




When the body is homogeneous and regulai*, how is this found ? 
tho centre of gravity of bodies homogoncous and irregular? 



How find 



CENTRE OF GRAVITY. 21 

from the -^ire of the stand (9), or by a string, the centre of 
gravity "will lie in the lines drawn from the wire or string per- 
pendicular to the horizon, and at the point where these lines 
intersect. 

13. K a body be not of uniform density, the centre of gravity 
will not be at the geometric centre, but at a point nearer to the 
denser edge. This may be illustrated by the circular body 4, 
Fig. 2, one side of which has been plugged with lead. In this 
case, if the body be placed upon the wire, with this passing 
through a hole at its geometric centre, the parts will not be 
balanced ; but, in order for this, will require the wire to pass 
through a hole at a point nearer the lead, as seen in the figure. 
If such a body be balanced and made to revolve, its apparent 
centre of gravity will seem to ascend at each revolution. By 
thus pluggiug one side of a wooden ball, it may be made to roll 
up an inclined plane, since its centre of gravity may thus be 
made to fall before the point of support. The same singular 
phenomenon may be exhibited by placing a double cone at the 
foot of an inclined plane, formed by two angular strips, as 
shown in Fig. 3, when the cones will be found to roll up the 



plane &om the angular point. This apparently contradictory 
motion is due to the inclination of the cones exceeding that of 
the inclined plane, causing the centre of gravity to fall con- 
stantly before the points of support. 

Two bodies, joined by a bar or rod, may be regarded as one 
body. If these be of equal weights their centre of gravity will 

If the body be not of uniform density where wiU its centre of gravity be ? 
Illustrate this by the figure. How may a wooden ball be made to roll up an 
inclined plane? Why does the double cone, seen in Fig. 3, roll up the inclined 
plane ? Case of two bodies joined by a bar or rod, as in the figure ? 



22 CENTRE OF GRAVITY. 

be a point midway between the two : but if of unequal weights, 
this will lie nearer the heavier body. This is shown by 3, 
Fig. 2. Here the ratios of the distances of the two bodies 
from the centre of gravity are inversely as the weights of the 
bodies. 

A body in a state of unstable equilibrium wdll be much less 
liable to be upset, if it have a rapidly revolving motion about its 
point of support ; for in this case the centre of gravity, although 
not directly over this point, is constantly revolving about a 
vertical line passing through it, and thus the tendency of the 
body to fall in a particular direction is prevented by a change 
in the point of support, so as to throw the centre of gravity in 
the opposite direction. 

From this cause a top spins on its unstable support. Many 
feats of public exhibiters, as the balancing of broad plates on 
the point of a sword, are performed by giving these objects a 
revolving motion as above. 

14. The equilibrium of a body is said to be stable whenever 
the perpendicular through its centre of 
gravity falls vnthin the base ; but in- 
7 different when this falls just at its 
edge^ and unstable when the perpen- 
dicular falls without the base. Thus, 
when the upper portion of the miniature 
tower, seen in Fig. 4, is removed, so as 
to bring its centre of gravity at «, where 
the perpendicular falls within the base, 
it will require considerable force to over- 
turn it ; but if the top be replaced, so 
as to bring the centre of gravity at 6, it 
will have an indifferent equilibrium, 
and be upset by the slightest force. By any addition to its 

Define three kinds of equilibrium. Illustrate these by Figure 4. • 




CENTRE OF GRAVITY. 



23 



height, so as to carry the centre of gravity above Z>, the struc- 
ture will be unsupported, and fall.* 

The equilibrium of a body may be either stable, indifferent or 
unstable. 

The firmness of a structure depends on the extent of the base 
upon which it rests, and the nearness of its centre of gravity to 
this base ; hence, of all artifical structures, the pyramid affords 
the best example of stability and permanency. A regard to these 
led the ancient Egyptians to build the tombs designed for pre- 
serving the embalmed remains of their princes, through indefi- 
nite ages, of the pyramidal form. 

Such a form of body is upset with difficulty, owing to the 
fact that its centre of gravity, being at a low point, must rise 
through a considerable curve, as the body turns on the edge of 
its base. From this cause, in turning over a flat stone or heavy 
marble slab, for instance, the force required at first is consider- 
able, but gradually diminishes as the centre of gravity rises and 
approaches the point directly over 
the point of support, where the 
equilibrium of the body becomes 
indifferent. 

Many amusing toys for children 
are constructed, illustrating the 
centre of gravity, where this is at 
or below the point of support. 
.Thus, in Fig. 5, a horse with his 
rider is supported on two small 
wires, projecting slightly from the 
hind feet, by means of the lead ball 



Fig. 5. 




* The leaning tower at Pisa has an elevation of three hundred and fifteen 
feet, and an inclination from the perpendicular of 12.4 feet ; and yet stands 
firmly, since the perpendicular, let Ml from its centre of gravity, comes within 
its base. 



What form of structure best insures permanency? In turning over a 
marble slab, why does the force required becomes less and less ? 



24 CENTRE OF GRAVITY. 

attached to the bent wire, whereby the centre of gravity is 
thrown below and under the point of support, causing the image 
to vibrate up and down from the slightest cause. 

From what has been said, we see the danger of loading 
wagons too high, and of piling too much baggage upon the top of 
stage-coaches, whereby the centre of gravity will be elevated, 
and these thus rendered more liable to be overturned. 

In the various movements of the human body, a constant 
effort is unconsciously made to support the centre of gravity. 
Since the feet afford a base so narrow, the centre of gravity 
would be constantly liable to fall without these, were it not for 
the counter motions of the arms, the head, etc., to prevent 
this. 

Thus, the child, before it has learned the wonderful art of 

walking, and the intoxicated person, who disregards it, are both 

subject to repeated falls. The rope-dancer, on the other hand, 

acquires such skill in balancing the body as to walk, dance, and 

perform various kindred feats, on a base so 

Fig. 6. narrow and unstable as a taut-rope. 

A person, in carrying a weight, as a pail 

^^^ of water (Fig. 6), for instance, in one hand, 

^^^ throws out the other arm, and inclines the 

^HBk. body, so as to bring the common centre of 

JIHv ^%K g^^^^^y ^f *^^s ^^^^ ^^® weight within the 

/MWr ^ base formed by the feet. For the same 

^iR| ^ reason, in walking up hill, the body inclines 

Tfi m ^S forward at an angle with the hill-side, and 

jp I M .^f ^^ walking down, is thrown backwards at a 

"" '^""^^ similar angle.* 

* A person, in running, inclines his body so as to throw the centre of gravity 
a little before the point of support, and thus aid is given to his forward motion. 

Why is there danger in loading wagons, etc., too high? The centre of 
gravity in the case of the human body ? Why is a child more liable to fall 
than an adult ? The case of rope-dancers ? Why does a person in carrying ;i 
pail of water extend the other arm ? 



MOTIOX OF BODIES. 



MOTION. 



15. Motion is a change in the position of the body, and is 
opposed to rest. The power which sets a body in motion is 
termed force. Motion is of several kinds. Relative motion is 
produced when a body is moving in respect to some one body, 
but at rest in regard to another. Thus, a man sitting upon the 
deck of a vessel, is in motion in respect to the land, but at rest 
in regard to the several parts of the vessel. Uniform motion 
is the motion of a body moving over equal spaces in equal times. 
Accelerated motion is produced when the spaces passed over in 
equal times increase, and retarded motion when these diminish. 
A stone falling through the air is an instance of the former, since, 
acted on by the force of gravity, its rate of motion constantly 
increases ; while the ascent of the stone, when thrown from the 
hand, affords an example of retarded motion. 

The velocity of a moving body is measured by the space 
passed over in a given time. A moderate wind has a velocity 
of about ^.b feet in a second ; a hurricane, of one hundred 
and eighteen feet in a second ; hence we say that the velocity 
of the latter is about eighteen times as great as that of the 
former. 

Since force is required to overcome the inertia of a body and 
give it motion, so the same is required to bring it to a state of 
rest when in motion. The chief forces which act to retard or 
destroy the motion of a body are friction and resistance of 
the air. 

In rising from a chair, we stoop for-ward, or bring the feet back, in order to 
cause the centre of gravity to fall within the point of support. Thus the move- 
ments of the body, in its various positions, all regard the law of equilibrium of 
solids. 

Define motion. Force. What is relative motion ? Uniform motion ? Ac- 
celerated motion ? Retarded motion ? Illustrations of accelerated and retarded 
motion ? How is the velocity of a moving body measured ? Illustration ? 
What are the forces which act on bodies to bring them to a state of rest ? 
3 



26 MOMENTUM. 

16. Mo77ienium. — This is the force which a moving body 
exerts, and is as the mass of the body multiplied into its veloc- 
ity. For equal masses of matter the momentums are as the 
velocities ; and for equal velocities, as the masses. Thus, a body 
weighing ten pounds, and moving with a velocity of five hun- 
dred feet in a second, will have a momentum of (10X500) 
five thousand, while a second body, also weighing ten pounds, 
and moving with a velocity of two hundred and fifty feet in a 
second, will have a momentum of only (10x250) twenty-five 
hundred. In this case the momentum of the former is double 
that of the latter. 

The velocity of a body may be very small, and yet have a 
very great momentum. This is illustrated in the case of ice- 
bergs or large timber-rafts, moving with a motion almost im- 
perceptible, and yet producing efiects the most terrific when 
meeting with other large masses. 

17. Gravity gives to falling bodies a constantly increasing 
or accelerated motion. 

If a person leap from a chair to the ground, he suffers no 
injury, while, if he do the same from a church belfry, he will 
most probably strike the ground with a force sufficient to de- 
stroy life. Now, since this force depends on the velocity with 
which the body moves at the moment when it touches the earth, 
it folloAVS that the velocity of the body is increased with the 
height. 

Laws of Falling Bodies. — Bodies in falling towards the 
earth, observe certain laws in regard to the rates of their veloc- 
ity. Thus, if a body fall sixteen feet in the first second of its 
descent, it will fall three times that in the next second, five 
times in the third second, seven times in the fourth second, and 
so on ; the spaces through which it moves in each successive 

Define momentum. Illustrate this. How does gravity affect the motion of 
falling bodies ? Example ? State the law in regard to the Telocity of falling 
bodies. 



LAWS OF FALLING BODIES. 



27 



second being as the odd numbers 1, 3, 
5, 7j etc. The total distance fallen 
through from the place of rest will be 
as the squares of the times. Thus, if 
a body dropped from an elevation fall 
16 feet in the first second, the distance 
reached at the end of the next second 
will be (2^) four times that, or 64 feet ; 
at the end of the third second, (3^) 
nine times, or 144 feet, and so on. 
Thus, to ascertain the entire distance 
a body will fall in a given time, we 
have only to multiply the space 
through which it fills in the first 
second by the square of the number of 
seconds it is falling. 

Attoood's Machine^ Fig. 7, is an 
ingenious yet simple device for ascer- 
taining the rate of increase in the ve- 
locity of falling bodies. This was 
contrived for the purpose of obviating 
the difficulties attending attempts to 
determine the velocities ' of these by 
actual measurements, and consists of a 
grooved wheel, jP, revolving on delicate 
bearings placed upon the top of a tall, 
graduated, vertical post. Over this 
wheel passes a fine cord, to the ends of 
which are attached the weights w^ s, 
precisely equal. To the front of this 
post is affixed a clock with its pendu- 



How will be the total distance fallen through 
by a body ? Explain this. How may we ascer- 
tain the entire distance a body has fallen in a 
given time ? What is the use of Atwood's Ma- 
chine ? Describe the parts of this. 



28 MOTION OF PROJECTILES. 

lum vibrating seconds. D and E are planes movable up and 
down on the post ; the latter having a circular opening just 
sufficient to allow the weight w to fall freely through it, while 
it intercepts and takes off a small oblong weight ~^ placed on 
this. 

Experiment. — Bring the post into a vertical position by 
means of the screws B B ; draw w to the top of the gradua- 
tion, place upon it the small weight, and let them descend ; the 
velocity of w will be comparatively slow yet constantly increas- 
ing until it passes through E, when the small weight will be 
taken off, and to will continue its descent to D, by reason of the 
gravitating force of the small weight acting down to the point 
E. Now, since gravity acts alike on all falling bodies, small 
and great, causing them to fall alike when the air offers no 
resistance, and since the increase of the velocity of w (during 
the successive intervals of time as indicated by the pendulum) 
is in the same proportion as that of a body falling freely during 
the same time, it follows that from the rate of increase in the 
velocity of lo^ as may be clearly indicated by this machine, the 
increasing velocity of bodies in general may be determined. 
The spaces passed over by the descending weight w. in succes- 
sive seconds, are found to be as the squares of the times of fall- 
ing. To determine, therefore, the distance a body has fallen in 
a given time, we have only to multiply the space fallen through 
in the first second by the square of the number of seconds. 

Owing to this accelerated motion of falling bodies, a bullet or 
a cannon-ball dropped from an elevation sufficiently great, may 
acquire a velocity and force far greater than if fired from a rifle 
or cannon. 

18. Motion of Projectiles. — When a body is thrown in a di- 
rection oblique to the perpendicular, it is acted on by two forces, 

* This small AYeight is not sliown in the figure. 

How does the slow descent of the weight w indicate the rate of velocity of 
falling bodies in general ? What is said of a bullet or cannon-baU let fall from 
a great elevation ? 



CENTRAL MOTION. 



29 




the projectile force, which tends to impel it forward in a straight 
line, and the force of gravity, which acts to bring it to the earth. 

Instead, therefore, of fol- 
lowing the direction of eith- 
er, the body describes a 
curve between the two 
forces. This may be illus- 
trated by Fig. 8. Let any 
body, as a cannon-ball, be 
projected horizontally from 
an eminence at a. Sup- 
pose h the point to which 
the projectile force alone 
would carry the body in one 
second, and h' the point 
which it would have reached 
by gravity alone in the same time ; then, instead of following 
the direction of either of these forces, it will move in a curve, a, 
r, between them. So, in the next second, instead of passing to 
the points c or c', under these combined forces it will move to 5; 
in the third second to ^, and so on. describing in its descent that 
form of curve known as the 'parabola. 

The law of motion of projectiles is especially regarded in the 
art of gunner y. By knowing the force of the powder which 
drives the ball, the engineer is enabled so to elevate his cannon 
or mortar as to cause the ball or shell to fall on a particular 
spot in the distance. 

19. Central motion is the motion produced by the revolution of 
a body about a fixed point : as when a ball attached to a string 
is made to revolve about the finger. Owing to the inertia of 
bodies causing them to persevere in straight lines, these, when 
revolving in a circle, tend constantly to recede from the centre 

What forces act on a body tlirown oblique to the perpendicular ? What di- 
rection will such a body take ? Explain Fig. 8. Where is the law of motion 
of projectiles especially regarded ? Define central motion. 

3* 



30 



CENTRAL AND CENTRIFUGAL MOTION. 




of motion, and fly off in a tangent to 
the circle. Thus, in Fig. 9, the body 
a, at the points a, 6, tends to move 
in the direction of the tangents a a', 
b Z>', and so at every other point in 
the circle. 

The force which holds a body and 
confines it in its circular path, is 
termed the centripetal force ; * that 
which causes it to fly off, the centrif- 
vgal-\ 

While these forces are balanced the body moves in a circular 
course, f but when either preponderates it moves in the direction 
of the preponderating force. This is seen in the revolution of 
an apple attached to a string fastened to the finger. If the mo- 
tion be too rapid, so as to break the string (centripetal force), 
the apple will fly off in a tangent ; but if too sIoay, it will fall 
in towards the finger. Owing to this centripetal force, a pail 
of water or a tumbler of water fixed in a sling may be made to 
revolve without the liquid flowing out. The same force causes 
water to fly off from a grindstone, or mud from a rapidly re- 
volving carriage- wheel. 

A magnificent illustration of the balancing of these two forces, 
so as to cause a harmonious revolution, is furnished in the 
motions of the heavenly bodies. 

20. The centrifugal force increases with the distance from 
the centre of motion. — Thus, in the revolution of a body about 

* Centre, and peto to seek. t Centre, and f^igio to fly off. 

X The danger of upsetting a carriage in turning rapidly round a corner is 
due to the fact that the centrifugal force causes the centre of gravity to be 
thrown without the wheels or point of support. 



Explain the causes of the circular motion of the body seen in the figure. 
What is centripetal force ? Centrifugal ? How does a body move when these 
forces balance ? Illustrations ? What bodies illustrate the balancing of these 
forces ? State the proposition in regard to the centrifugal force. 



ACTION AND REACTION. 



31 



^:^^5i:a 



a fixed axis, as the earth for instance, those parts more remote 
are acted on by the centrifugal force much more strongly than 
those nearer this axis. To this cause is commonly ascribed the 
present form of the earth, bulging out at the equator and flat- 
tened at the poles : this having commenced its revolution at an 
early epoch, probably while it was in a plastic state. 

Til is change in the form of revolving bodies may be illustrated 
Fi?-io. by Fig. 10. This consists 

of two elastic hoops fast- 
ened at the upper side on 
a vertical shaft, while the 
lower is free to move. 
Rapid motion is given to 
this by geared wheels at 
the top of the frame, when 
the hoops will be found 
to bulge out at the equa- 
tor, but contract at their 
^ #' poles, as already stated. 

"When a body in motion strikes against another body, the 
former acts upon the latter, and in turn is acted on by it. The 
effect of the moving body is 
termed action^ and of the body it 
strikes, reaction. Action and re- 
action are equal and in opposite 
directions. This may be proved 
by the apparatus shown in Fig. 
11, where six ivory balls are sus- 
pended by fine cords so as to hang 
parallel before a graduated arc. 

^ ' ' " ' '""'^ If, now, one of the balls be drawn 

back to a certain point on the arc, and then let fall, it will act 





Cause of the present form of the earth ? How illustrated by the figure ? De- 
fine action and reaction. What law do these observe? Explain Fig. 10. 



32 



RESULTANT MOTION. 



on the second of the series, this second ball will act on the third, 
the third on the fourth, and so on ; each imparting force to the 
next, until the last ball, having no other to which to impart its 
force, flies off to nearly the same distance as the first was re- 
moved ; thus showing action and reaction to be equal where the 
bodies are perfectly elastic. 

21. Reflected Motion. — When any elastic body, as an ivory 
ball, is thrown against a hard smooth surface, it rebounds from 
such surface, and the motion it receives is called reflected mo- 
tion. In such a case, the angle which the body makes with the 
surface when approaching it is called the angle of incidence^ 
and that which it makes with this when leaving it, the angle 

of reflection ; these angles 
are equal. 

Thus, let an elastic ball 
be thrown from the point 
a, Fig. 12, striking the 
hard smooth surface d, e, 
at b, when it will be re- 
flected in the direction b, 
c. In this case, a, b, d, is the incident, and c, b, e, the 
reflecting angle, and upon measuring these angles they will be 
found equal. 

^igi3. 22. Resultant Motion. 

— "When a body is struck 
at the same instant by two 
forces, whether equal or 
unequal, instead of follow- 
ing the direction of either 
it will move in a line be- 
tween these forces ; this line 
or direction will be the 
diagonal of a parallelogram whose adjacent sides represent 

What is reflected motion ? How are the angles of incidence and reflection ? 
Explain Fig. 12. What is resultant motion ? Illustrate this by the figuro. 





RESULTANT MOTION. 



33 




the forces, and will itself represent the resultant of these 
forces. Let a, Fig. 13, be a billiard ball, for instance, struck 
by two unequal forces in the directions 6, c, and d^ e ; the ball, 
instead of following the direction of either, will move in the 
diagonal a, f, between the forces.* 

Pig. 14- The scenes of daily life 

afford numerous examples of 
resultant motion. A vessel 
in attempting to cross a riv- 
er presents a good illustra- 
tion. Thus, in Fig. 14, let 
B be a steamboat attempt- 
ing to cross the stream in a 
direct Line to A'. Suiopose 

* Fig. 15 is a convenient apparatus for showing the resultant of two forces 
acting simultaneously on a body. Two springs, b and d, are placed at one 

corner of a rectangular table, so as 
-^^°' ^^' to act equally on a ball, a, if de- 

g /'sivp.d. If 6 or rf be worked sep- 

arately, it will impel the ball in a 
straight line in the direction of the 
forces to c or e ; if both these act 
equally, and strike a at the same 
instant, it will move in the diago- 
nal a,f, between the forces. If 
the springs be drawn out so as to 
act unequally on a, they may be 
so regulated as to drive it in any 
direction between c,f, and e,f. 

Many wonderful feats of circus- 
riders are examples of resultant 
motion ; such as jumping from the 
back of a horse in rapid motion, 
over a rope or through a hoop, so 
as to alight again on the back of the animal. The spring of the body in such 
instances gives to it an upward motion, while it retains its forward motion in 
common with the horse. 




How is resultant motion illustrated in case of a steamboat attempting to cross 
a river ? 



34 RESULTANT MOTION. 

the force of the steam sufficient to drive it to A' in ten min- 
utes, while the force of the current is sufficient to carry it in the 
same time to A. If, now, steam and current act simulta- 
neously, instead of passing to either A or A', the steamboat will 
move in a diagonal between the two forces, and arrive at the 
expiration of time at B'. 

In throwing a package from a train of cars, or an apple from 
the deck of a steamer, these bodies partake of the motion of the 
cars or steamer, so that, instead of striking at the point intended, 
they are carried along some distance beyond. Thus, in firing 
from a sailing-vessel at an object at rest, due allowance should 
be made for the motion of the vessel, and aim taken behind the 
object. 

In j.umping upwards from the floor of a rail-car in rapid mo- 
tion, a person unacquainted with the laws of resultant motion 
might very naturally suppose the car would pass from under 
him so as to cause his feet to strike again at a point behind that 
from which he jumped, instead of returning to this same point, 
as is found to be the fact. So, in dropping a ball from the mast- 
head of a vessel sailing rapidly, an ignorant person might sup- 
pose the ball to strike astern, instead of at a point on the 
deck directly beneath, as the trial will prove. 



PRACTICAL PROBLEMS ON THE FOREGOINO 
PRINCIPLES. 

1. What distance would a pigeon, flying uniformly at the rate 
of 68 miles per hour, pass over in 12 J hours ? 

2. A train of rail-cars which, with the locomotive, weighed 180 
tons (403,200 lbs.), and moving at the rate of 18 miles per hour, 

In throwing a package from a rail-car, or the deck of a steamer, will it 
strike the point at which it is aimed ? In firing from a sailing vessel at an 
object at rest, where should aim be taken ? In jumping upwards from a rail- 
car, when moving rapidly, why does not the car pass from under the person ? 



PROBLEMS. 35 

was met by a second train weighing 165 tons (369,600 lbs.), and 
moving at the rate of 22 miles per horn.- ; what was the force or 
momentum of their collision? (§ 16.) 

3. A grindstone in an axe factory burst asunder from its too 
rapid revolution, and a portion weighing 428 lbs. was hurled against 
the wall with a velocity of 30 feet per second ; with what force did 
it strike this ? 

4. A stone, let fall from a precipice above a body of water, was 
seen to strike this in five seconds; what was the elevation above the 
water? (§17.) 

[Suppose 16 feet to be the distance flillen through in the first 
second; then 16 X 5' = 400. Ans.] 

5. A ball shot perpendicularly upwards from a rifle returned in 
12 seconds ; how high did it ascend ? 

[6^ X 16 = 576. Ans.] 

6. If a body be hurled downwards with a velocity of 22 feet in 
the first second, how far will it fall in 9 seconds ? 

[Since the total distance fallen through by a body will be as the 
square of the time of its fall (§ 17), the answer in this example 
will be found by multiplying 9'^ (81) by the distance fallen through 
in the first second.] 

7. A stone, thrown directly at an object from a rail-car moving 
at the rate of 3,520 feet per minute, was 4 seconds in the air ; at 
what distance beyond the object did it strike ? 

8. A cannon-ball fired from a steam frigate was seen to strike a 
fort three miles distant in 1^ seconds ; supposing the frigate to be 
moving at the rate of 15 miles per hour, how far behind the fort 
would it be necessary to aim in order that the ball might strike it? 

9. Two steamboats moving in opposite directions pass each other, 
one going at the rate of 11 and the other 14 miles per hour ; suppose 
an apple be thrown from the deck of one boat at a person in the bow 
of the other boat, how far astern will it strike the water in 3i 
seconds? 

10. In carrying a heavy package along a narrow pass upon the 
verge of a promontory on the island of Atoi, several years since, a 
man was seen to lose his balance and fall, and in 4i seconds after to 
strike the water beneath ; how high was the promontory ? 



36 MECHANICAL POWERS. 



THE MECHANICAL POWERS. 

23. Machines are instruments employed for aiding muscular 
and other forces in overcoming physical resistances. These, 
however complicated, are made up from a few simple machines, 
termed the Tiicchanical powers. It must not be supposed that 
we generate or increase force by means of these simple 
machines ; we merely apply this in a convenient and economic 
manner. Thus, if a man could raise to a certain height two 
hundred pounds in one minute, w^ith the utmost exertion of his 
strength, no power could enable him to raise tw^o thousand 
pounds in the same time. If left to elevate this mass by his 
own unaided strength, he would be obliged to divide it into ten 
equal portions, and raise each separately ; whereas, by making 
use of one of the simple machines, he will be enabled to raise 
the wdiole mass at once, requiring, however, for the performance 
of tbe task, ten times as many minutes as for raising the two 
hundred pounds.* Thus, in the use of these simple machines, 
time is exchanged for power. And the same is true of all the 
numerous varieties of mechanical contrivances for facilitating 
labor. 

24. The mechanical powers, or simple machines, are com- 
monly regarded as six in number : the Lever, the Wheel and 
Axle, the Pulley, the Inclined Plane, the Wedge, and the Screw. 

The Lever. — This is any bar, turning on a fixed point 
or prop. This prop is termed the fulcrum,^ and the parts 
of the bar extending on each side of this the arms of the 
lever. 

Levers are of three kinds. Firsf, where the fulcrum is 
between the power and the weight. Seco?id, where the weight 

* Bird. 

What are machines ? From what are these made up ? Do we generate force 
by the use of machines ? Illustrate this. The number and names of the 
mechanical powers ? Define the lever. The kinds of levers ? 



THE LEVER. 



37 




Ml 



is between the power and fulcrum. Third^ where the power 
is between the fulcrum and weight. 

j,.„ ^g The first kind of lever is 

shown in Fig. 16, where P rep- 
resents the power, W the weight 
or resistance, and F the fulcrum. 
The crowbar employed for 
prying up rocks, pump-handles, 
steelyards, scissors and pincers, are illustrations of this kind 
of lever. 

The power and weight are always in equilibrium when they 
are to each other in the inverse ratio of the arms of the lever 
to which they are attached. Consequently, in Fig. 16, in order 
that the power, P, and the weight, W, exactly balance each other, 
the products arising from multiplying each by its distance from 
the fulcrum must be equal. Thus, if we suppose W to be a 
weight of three hundred pounds placed two feet from F, and 
P a power of one hundred pounds placed six feet from F, then 
will W and P be in equilibrium, for (300 X 2) == (100 X 6). 
Thus, when the weights and lengths of the two arms of the 
lever are given, the power to balance the weight may be found 
by dividing the product of the weight into its distance from the 
fulcrum by the distance of the power from the same. 

The balance and steelyard are examples of the application 
of the first kind of lever. The 
latter, as seen in Fig. 17, has 
the length of its arms unequal. 
The body to be weighed is sus- 
pended from a hook near the ful- 
crum, and counterpoised by a 
small weight, moveable upon the 
other, or long arm ; the weights 
which this small weight will bal- 

Describe a lever of the first kind. When are the power and weight in equi- 
librium ? Illustrate this by the figure. What familiar examples of the first 
kind of lever ? 

4 



Fig. 17. 





do THE LEVER. 

ance at different points, being indicated bj figures marked on 
this long arm. By this inequality in the two arms of the steel- 
yard the weights of heavy bodies may be indicated. 

25. Fig. 18 represents a lever of the second k'md. Here 

the fulcrum, F, is at one end of the 
^^' ' lever; the power, P, acting at P at 

the other, while the weight, W, is 
between these. The power and 
weight will be in equilibrium in 
this as in the previous case, when 
(W X wF) = (P X PF). An 
oar employed in rowing a boat is 
an example of this kind of lever ; in this instance the water 
is the fulcrum, the boat the weight or resistance, and the 
hands the power. Wheelbarrows, doors, hay-cutters, and nut- 
crackers, are also examples of the same. 

26. The third kind of lever may be illustrated by Fig. 19, 
Fig. 19 where W, the weight attached at w, 

is at one end ; F the fulcrum, at the 

other, and P the power applied at P 
£- between these, when the power and 

weight balance (w F X W) = 

(p F X P). 

As will readily be seen, the power 

of this form of the lever must always 
exceed the weight to be raised. Hence, owing to its mechanical 
disadvantages, this lever is never used, except where velocity 
is required more than power. A pair of tongs, the treadle of a 
lathe, and the raising of a ladder by lifting upwards when one 
of its ends is fixed, afford illustrations of levers of the third 
kind. 

Explain the second kind of lever from the figure. Examples of this kind 
of lever ? Explain from the figure the third kind of lever. In this kind of 
lever, how must the power be in reference to the weight ? 



(^ 



COMPOUND LEVER. 39 

The limbs of animals afford the most striking examples of 
this form of lever. Here the tendons, which connect with the 
muscles which move the limbs, are attached, as in the fore-arm, 
to the bone near the joint on which the limb turns. Thus, a 
short yet powerful muscular contraction at the hips and 
shoulders gives the sweep to the legs and arms from which the 
body derives so much activity. 

27. The Compound Lever is a combination of simple levers 
acting one upon the other, whereby the power of a small force 
in overcoming a large resistance is greatly multiplied. Such 
an arrangement is shown in Fig. 20. Here A B represents a 

Fig. 20. 



B r- 




^ 



lever of the first kind, which acts on a second lever. B C, and 
this again on C D, a third lever ; thus enabling a small force at 
A to overcome a large resistance at D. To ascertain the 
weight or force which a given weight or power at A will balance 
at D, we have only to multiply together the numbers express- 
ing the lengths of the arms upon the left of the three fulcrums, 
and this j)roduct into the power at A, and then divide the re- 
sult by the product arising from the continued multiplication of 
the numbers denoting the lengths of the arms on the right. If. 
for instance, the arms of the levers upon the left, be 6, 6, 6, 
and those on the right 2, 2, 2, and the power at A one pound, 
then if (6 X 6 X 6 X 1) ^ (2 X 2 X 2) = 27 pounds, the 
weight which one pound at A will balance at D. * 

■* la the various calculations in meclianics, the levers are regarded only as 
imaginary lines, and no account is taken of friction, etc. ; these causes mod- 
ify somewhat the practical results. 

Examples of the third kind of lever ? What is a compound lever ? Explain 
this from the figure. 



40 



WHEEL AND AXLE. 






The knee lever, hay-scales, etc., are instances of the applica- 
tion of the compound lever. 

28. The Wheel and Axle. — In raising a weight to any 
considerable height, by means of the common form of lever, it 
is necessary for this to act through a succession of interrupted 
movements. To avoid this, and enable the power to act by a 
continuous motion, the wheel and axle are employed. This, 
then, may be regarded as a modified form of the lever. 

This machine is shown by Fig. 21, where W represents the 
wheel, and A a, the axle. The 
power, P, acts by means of a 
rope passing in a groove upon 
the circumference of the wheel. 
This wheel is oftentimes sup- 
plied by levers fixed in A, to 
the ends of which the power is 
applied. The resistance or 
weight, B, to be raised, is at- 
tached to a rope which winds 
around the smaller cylinder, a. 
Here the radius of the larger 
wheel corresponds to the long 
arm, and that of the smaller 
wheel or axle, to the short arm 
of the lever. Accordingly equi- 
librium is obtained when the 
power applied is to the weight to bei, raised, or resistance to be 
overcome, as the radius of the axle is to that of the wheel. If K. be 
the radius of the wheel, and r that of the axle, then the power, P, 
and weight, B, will be in equilibrium, when P X R = B X r. 
So also the weights Z», and B, will be in equilibrium when the 




Instances of the application of the compound lever ? In what cases is the 
■wheel and axle used, and how may it be regarded ? Explain its operations by 
the figure. When will the power and weight be an equilibrium ? Wliat is 
the windlass ? 



WHEEL AND AXLE. 



41 



product of 5, into the radius of the larger axle, A, is equal 
to that of B, into the radius of the smaller axle, a. The 
ivindlass^ seen at W, Fig. 22, is a form of the wheel and 
axle, in which the power is applied to levers fixed in the circum- 
ference of the wheel. In this case, the rope attached to the 
weight, when this is raised, winds off from a smaller upon a 
larger axle. In this manner a great weight may be raised by 
a small power, and with a simple machine. Such a form is 
usually termed the differential wheel and axle. 




The Capstan is a form of the wheel and axle usually em- 
ployed on shipboard for raising anchors, or upon wharves for 



4* 



Describe the capstan. 



42 THE PULLEY. 

moving vessels and other ponderous bodies. This is usually 
placed in a vertical position, and has levers fixed in its larger 
circumference, or temporarily inserted in holes provided for 
these. The form of the capstan, with the cable winding upon 
this, is shown at H, Fig. 22. 

Where great power is required, or where it is desired to 
transmit the power in another direction, a combination of 
wheels, connected bj cords or bands, or bj teeth upon their 
circumferences, is employed. Such an arrangement acts on 
the principle of the compound lever, already described. 

29. The Pulley^ strictly speaking, consists of a rope or 
cord sliding over a fixed point or cylinder, and acts upon the 
principle of the lever. As commonly understood, however, this 
consists of a rope moving over a grooved wheel, which turns on 
pinions, the wheel being introduced to prevent friction. The 
use of the pulley gives no increase of power, but simply affords 
a convenient mode of applying power. 

Pulleys are divided mio fixed and Qnovahle. At I, Fig. 22, 
is seen the simple form of the fixed wheel-pulley. Here no 
power is gained, but merely a convenient application of power 
in the moving of bodies is afforded. Thus, if a weight is to be 
raised from the ground up to the point I, it may be done with 
far less difficulty by a person pulling doion upon the rope pass- 
ing over a wheel, than if the same person apply his power at I 
in pulling up on this rope. So, in raising bales of goods and 
heavy merchandise to the upper stories of warehouses, or from 
the holds of vessels, the pulley affords a highly convenient 
means of changing the direction of the power, and so facilitat- 
ing the expenditure of labor. 

Of what does the pulley, strictly speaking, consist ? Of -what does it con- 
sist as commonly understood ? What advantage does the use of the pulley 
afford ? How are pulleys divided ? Explain the first or simple form of 
the pulley. Where is this form employed ? Is there any power gained 
by it, and, if not, what are the advantages of its use? 



THE INCLINED PLANE. 43 

At J, Fig. 22, is shown a fixed and a movahle pulley. 
Here the weight attached to the lower blocks is supported by 
the two parts of the cord which passes around the wheel of this 
block ; and consequently the power upon the free end of the 
cord requisite to balance this weight of 4, will be 2. Hence, in 
this, as in the combinations at K and M, the power will be to 
the weight it will balance as 1 to the number of cords sup- 
porting the lower block. Thus, if 8 represent the weight at 
K, and the number of cords which support this weight be 
three, then, since each cord will sustain one third of this, the 
power at the free end of the cord requisite to maintain the 
equilibrium will be one third the weight. So at M, where the 
weight is supported by six cords, the power to equilibriate with 
this will be one sixth the weight. 

From its portable form, cheapness of construction, and the 
facility with which it may be applied in almost every situation, 
the pulley is one of the most useful of the simple machines. 
The mechanical advantage, however, which it appears in 
theory to possess, is considerably diminished in practice, owing 
to the stiffness of the cordage and the friction of the wheels 
and blocks. From these causes it is computed that in most 
cases two thirds of the power is lost. The pulley is much 
used in building, when weights are to be elevated to great 
heights; but its most extensive application is found in the 
rigging of ships, where almost every motion is accomplished by 
its means. 

30. The Inclin ed Plane. — If a person, in attempting to raise 
a heavy body, as a cask for instance, find its weight unequal to 
his strength, he may accomplish his object by causing the body 
to be supported in part by an inclined surface, and exerting his 
force in a direction parallel to this inclined surface. Thus, in 

Explain tlie fixed and movable pulley. In these combinations of pulleys 
how is the power to the weight ? What is said further of the use of the 
pulley ? Where is it much used ? Illustrate the use of the inclined plane for 
facilitating the raising of a heavy body. 




44 THE INCLINED PLANE. 

loading a barrel into a wagon, a plank is laid with its lower end 
resting upon the ground, and the barrel rolled up this by a 
force much less than would be required to. raise it perpendicu- 
larly to the same elevation. The plank in this case forms an 
inclined plane. 

Fig. 23 presents a view of this simple machine, by which 
the weight upon the truck is 
raised to a given height by a 
force comparatively small. 
Here it will be readily seen 
that the inclined surface sus- 
tains the greater portion of the 
' weight, while the power act- 
ing in the direction of the 
■^ plane sustains the remainder. 
In Fig. 23, A B is the length of the inclined plane, B C its 
base, and the distance between A C its height. If the weight 
placed upon the inclined plane consist of as many pounds as 
there are inches in the length of the plane, the pressure on the 
plane will be equal to the number of inches in the base, and 
the tendency to move down the plane will be balanced by as 
many pounds as there are inches in the height ; so that the 
force requisite to draw the body up an inclined plane will be to 
the weight as the height compared with the length. 

In this as in other simple machines a gain in power is always 
attended by a corresponding loss of time. This is seen in 
roads, which when not level may be regarded as inclined planes. 
If a road be made to pass directly up the side of a steep hill, 
a far greater power, but a much shorter time, will be required 
to draw a loaded wagon to its summit, than if the road wound 
around the sides of the hill at a less ano;lc of inclination. 



Describe this simple machine from the figure. In raising a body up an 
inclined plane, how will the power be to the weight ? In the use of the in- 
clined plane, with what is the gain in power attended? Give an illustration. 



THE WEDGE. 45 

31. The Wedge. — If, instead of moving a load on an in- 
clined plane, the plane itself be moved beneath the load, it 
then becomes a wedge. Thus, if a heavy beam be secured in 
a vertical position, and be free to move upwards and downwards, 
but not laterally, and an inclined plane, on which its end rests, 
be forced under it, the beam will rise by the motion of the 
plane merely. Such an inclined plane becomes a wedge. 

This simple machine is formed by two inclined planes laid 
base to base, as seen in Fig. 24. 

The power requisite to force the wedge beneath 
a resisting body will increase with the increase of 
the angle of inclination of its sides. 

The wedge is used in the arts and manufactures 
where immense force is required to move a body 
through a very small space. It is, therefore, used in 
raising vessels in docks, when about to be launched, 
by driving under their keels ; and also in oil-mills, 
for forcing out the oil from seeds, which are placed in 
bags between the plates of the press. It is chiefly 
used, however, in cleaving logs and masses of stone. 
Cutting and piercing instruments, as knives, shears, 
awls, etc., act on the principle of the wedge. The sides of these, 
where the power acts continuously, should form with each other a 
smaller angle, or be sharper, than where the wedge is driven by 
percussion, as in the splitting of timber, rocks, etc. 

32. The ScreiD. — When a road, instead of leading up a 
hill directly, winds round to its summit so as to lengthen the 
inclined plane, and thus aid the moving force, this inclined plane 
becomes a screw. In this manner a pair of stairs, winding 
around the sides of a cylindrical tower, either within or without, 
affords an instance of an inclined plane so modified as to become 

Illustrate the principles on which the wedge acts. How is the wedge 
formed ? What relation has the moving power of this to the inclination of its 
sides ? Where is the wedge used ? On what principle do cutting and pierc- 
ing instruments act ? Give illustrations of the screw. 



46 



THE SCREW. 



Fig. 25. 



a screw.* Thus it will be seen that the screw is but another 
form of the inclined plane. This may be best illustrated by 
winding about a small cylinder a strip of paper, cut at an angle 
so as to represent such a plane, and then tracing the course of 
its edge. This will be found to mark the direction of the 
thread of a screw, thus showing this to be but a winding in- 
clined plane. 

The form of the screw and course of the thread are shown 
by Fig. 25. 

The screw is not applied directly to the resistance to be over- 
come, as in the case of 
the inclined plane or 
wedge, but acts upon 
this through the screw- 
box, or nut. This is a 
cylindrical cavity, hav- 
ing a spiral groove 
cut around its interior 
surface to correspond 
with the elevations of 
the screw, and in which these move. This groove of the nut 
is termed the interior screw, and the elevations of the cylinder 
moving in this, the external screw. 

33. Power is commonly applied to the screw by means of a 
lever, either attached to the nut or to the head of the screw, as 
seen in Fig. 25. By varying the length of this, the power may 
be indefinitely increased at the point of resistance. 

Where the power is applied to the end of a lever attached to 
the screw, the comparative velocities of this and the weight or 

* Olmsted. 




Of what machine is the screw a modified form ? How illustrated in case of 
a cylinder and strip of paper ? Tlu'ough what does the screw act upon the 
resistance ? Describe the screw-box or nut. How is power commonly ap- 
plied to the screw ? Where the lever is applied to the screw, state the relation 
between the velocities of the power and resistance. 



MACHINERY. 47 

resistance will be as the circumference of the circle described by 
the povN^er to the distance between the contiguous threads of 
the screw. The same ratio of motion will also constitute the 
ratio between the power and weight when these are in equi- 
librium ; and hence, the longer the lever, and the nearer the 
spirals of the thread, the greater will be the mechanical force 
exerted by the screw. 

The screw is generally used where great pressure is to be ex- 
ercised through small spaces ; hence its agency in most presses. 
Thus, in the coining of money, where a prodigious force is re- 
quired to impress the die upon the metal, the screw is employed. 
So, also, in compressing cotton, hay, and other light and bulky 
bodies, in order that they may occupy the least possible space 
in transportation, this same machine usually acts a part. 

MACHINERY. 

34. All machines, however complicated, are only applications 
of some one or more of the simple mechanical powers just de- 
scribed. Thus, if we examine carefully the various parts of a 
complex machine, as a steam-engine, or a loom for weaving, we 
shall find these formed of simple levers, wheels, screws, etc., 
combined in various ways to make up the entire whole. 

The use of machines is not to create force, but merely to 
afford a means of applying this to advantage. This is obvious 
from the fact that these are mere inert matter, and incapable 
of doing more than to transmit the force or power imparted to 
them. In the use of the windlass for raising water from a 
well, or coal from a mine, the application of the power to the 
crank may be made with far greater convenience than to the 
rope winding upon the axle, and to which the weight is attached. 
So, in the rigging of ships, or the raising of materials for build- 
ing, a pulley is interposed between the power and the weight, 

Where is the screw generally used ? Of what are all machines composed ? 
The use of machines ? Give an illustration. 



48 USE OF MACHINES. 

not to increase the force of the former, but merely to give it a 
convenient and efficient direction. Whatever form of machine, 
therefore, be introduced between the power and resistance, it 
can add no mechanical energy to the former, but will, in fact, 
owing to friction and other causes of resistance, intercept a 
portion of the action of this power while transmitting it from 
the point of application to its working point. 

35. Machines serve to give intensity^ direction^ and veloc- 
ity to the 'power. — To raise a weight of one thousand pounds 
by an unaided muscular force of two hundred pounds, would be 
impossible ; but, by interposing a lever, or a screw and lever, in- 
tensity may be given to this force, whereby the weight shall be 
readily raised. 

In a windmill the power is the wind, which acts with a con- 
tinuous rectilinear force on the arms, while the stones, which are 
the resistance, revolve with a circular motion around a vertical 
axis. Here, as will be readily understood, the change in the 
direction of the force transmitted from the power to the resist- 
ance is effected by the use of a series of wheels. So. in a saw- 
mill, the force of the water communicates a rotary motion to 
the wheel, and this motion, by means of a crank, is converted 
into a reciprocating motion, as seen in the ascent and descent 
of the saw. 

Again, if a power having a certain velocity be required to 
impart a greater or less velocity to the resistance, then a ma- 
chine must be interposed w^hich will regulate this velocity in the 
required proportion. This is seen in the pulleys of a turning- 
lathe, or the wheels of a clock. 

36. The power of a moving force is expressed by the weight 
it is capable of sustaining. Thus, if a man, by his unaided 
strength, be able to raise a weight of two hundred pounds, he is 

Do machines add to the power ? What purpose do machines serve ? Illus- 
trate this in case of raising a weight. In case of wind and saw mills. State 
the use of machines in regulating velocity. How is the power of a moving 
force expressed ? Give illustrations. 



PLY-WHEEL. 49 

said to have a power equivalent to this -weight. So, if a steam- 
engine be equal to overcoming a resistance of fifty tons, it is 
said to have a povrer of fifty tons. 

The mechanical force or momentum of a weight, is deter- 
mined by multiplying it into the space through which it moves. 
So, also, the mechanical force or momentum of a power of any 
description may be ascertained by multiplying its equivalent 
weight into the quantity of its motion. When the momentum 
of the power acting through a machine is greater or less than 
that of the weight, the motion is accelerated or retarded in the 
direction of the power ; but when the momentum of the power 
equals that of the weight, equilibrium is maintained between 
these. 

Thus it will be understood what is meant when it is said, in 
the use of machines, poicer is always gairied at the expense 
of time ; for if, for instance, a small power act against a great 
resistance, the motion of the latter will be just so much slower 
than that of the power as the resistance or weight is greater 
than the power. 

37. Regulation of Force in Machines. — In order for ma- 
chines to operate successfully, it is necessary that their motions 
should be as uniform and regular as possible. For insuring 
such uniformity and regularity, various ingenious contrivances 
have been invented. 

The Balance or Fly Wheel affords a common and ejQfect- 
ual method of equalizing motion, especially in the heavier 
kinds of machinery. This usually consists of a heavy cast-iron 
wheel, fixed on the shaft near the crank, where the power of 
the engine or other force is applied. This balance-wheel serves 
as a magazine or repository for motion, overcoming by its mo- 
How is the momentum of a weight determined ? The result when the mo- 
mentum of the power is greater or less than the weight ? What is meant when 
it is said that power is always gained at the expense of time ? What is requi- 
site for the successM operation of machines ? Describe the balance-wheel and 
its use. 

5 



50 



THE GOVERNOR. 



mentum, any slight irregularities in either the moving power or 
resistance. 

38. The Governor is a highly ingenious device long since 
adopted for regulating mill-work, by regulating the quantity of 
the flow of water moving this. Its chief application has been, 
however, more recently to the steam-engine. 

26, where B B are two 
heavy balls, at- 



This contrivance is shown in Fig. 

Fig. 26. 




tached to the ends 
of rods, B F, B 
F', jointed at E, 
and passing through 
a mortice in the 
vertical shaft, S S'. 
As this shaft re- 
volves, centrifugal 
force causes the 
balls to fly out 
from it or spread 
themselves. At the 
same time the joints 
at E F' recede from each other, causing the ring at R, to 
which the arms, F R, F R, are attached, and which is movable 
on the shaft, S S', to be drawn down. A lever, attached to the 
ring at R by a joint, has its end depressed as the ring descends 
on the shaft. This acting over the fulcrum at H, and through 
the joints at I and J, upon a second lever connected with the 
valve V, placed in the steam-pipe, causes this valve to open less 
or more, according to the distance at which the balls revolve 
from the shaft. At W is a grooved wheel fixed upon S S'. 
A cord leading around this connects with another grooved wheel 
on the main shaft or axle, whereby a speed is always transmitted 
to S S', proportionate to that of the machinery. 

Thus, as the speed of the machinery increases, the balls sep- 



Describe the Governor. 



THE FUSEE. 51 

arate, the end of the lever at R is drawn down, and V closes, 
shutting off in a, proportionate degree the steam, or other motive 
power. As this power diminishes tjje balls, B B, approach each 
other, B rises, and V opens accordingly; so rendering the 
power just requisite for giving a uniform and regular motion to 
the machinery. 

39. The Fusee^ in watchwork^ is an instance in which a 

variable power is made to com- 
municate a uniform motion. 

Ji This may be effected by causing 

the velocity or leverage to in- 
crease as the intensity of the 
" ° power diminishes. Let B, Fig. 

27, represent the main-spring of a watch coiled up in a barrel, 
and connected with the fusee, A, by a fine chain. When the 
watch is first wound up, by winding the chain upon A, the 
spring acts with its greatest energy ; but then the leverage of 
the wheel, which is its semi-diameter at the point where the 
chain unwinds from it, is least, being at the smaller end 
of A. This causes the chain to wind off from the spirals 
of the fusee upon the barrel slowly, increasing its rate as the 
diameter and leverage of A increase, and the force of B 
diminishes. • 

Thus, with a varying force, the revolution of the fusee is 
made uniform. 

40. The Pendulum is a plummet or any heavy body sus- 
pended by a thread or small rod from a point of support, and, 
when disturbed, free to move about such a point as a centre. 
If a pendulum be drawn aside from its perpendicular or place 
of rest, and let fall, it will continue to vibrate in a vertical 
plane for several minutes, or even hours, until brought to rest 
again by the resistance of the air and friction at the point of 
support. 

Describe the Fusee. What is a Pendulum? Describe the operation of this. 



52 



THE PENDULUM. 



Fig. 28. 




Let P", Fig. 28, represent a pendulum suspended from the 
point C. If we bring this pendulum 
bqipk to the point P, and let it fall, 
it will describe the arc P P', reach- 
ing P" with such a velocity as to be 
carried forward and rise upon the op- 
posite side of the perpendicular to 
P'; from this point it again falls, 
traversing the arc P' P" P, and so 
continues its vibrations ; each vibra- 
tion describing a smaller arc than the 
previous, until the pendulum come to 
rest at P". 

The motion from P to P', and 
from P' to P, is termed a vibration, 
or oscillation^ and from either of these points to P", a semi- 
oscillation. 

41, All the Vibrations of a pendulum of the same length 
are performed in equal times. — Thus, in Pig. 28, if the ball 
P be let fall from that point at the same instant a second ball 
is set free at a, the declivity through which the former falls will 
be greater than that through which the latter moves ; conse- 
quently, its accelerated force on reaching P" will carry it forward 
through its vibration to F, in the same time that the ball at 
a moves through its vibration to a ; thus each ball will per- 
form its vibrations through different arcs, equally distant from 
the point of support, in the same time. ThiS; however, is not 
strictly true where the arc exceeds a certain limit, about 6°. 

The times of the vibrations of pendulums of unequal 
lengths are as the square roots of these lengths. Suppose, 
in Fig. 28, two balls attached to the common centre, C, be let 
fall at the same instant from the points P and d ; these will 

In the figure •what constitutes a vibration, and "what a semi-vibration ? 
How do pendulums of the same length vibrate ? State the proposition in re- 
gard to the times of the vibrations of pendulums of unequal lengths. Illus- 
trate this. 



THE PENDULUM. 53 

perform their vibrations through the arcs P P', and d d\ in 
une(^ual times. If d be four feet, and P nine feet from C, then 
will the times of their vibrations be as the square roots of these 
numbers ; so that while d performs three vibrations, P will 
make only two. 

42. The pendulum has been employed for determining the 
figure of the earthy and as a standard of weights and meas- 
ures. It is the accelerating force of gravity which produces 
the vibrations of the pendulum ; accordingly, the rate of these vi- 
brations will increase with the increase of this force. Pendulums 
of the same length are found to have different rates of vibration 
at different points upon the earth's surface, and to vibrate in 
equal times require to be of varying lengths. Thus, a pendulum 
which performed its vibrations in one second at Paris, was found 
to require lengthening .09 of an inch in order to perform its 
vibrations in the same time at Spitzbergen. This variation is 
caused by a variation in the force of gravity, and this force of 
gravity is found to vary with the distance from the earth's cen- 
tre (§ 11) ; hence, by means of the pendulum measuring the 
force of gravity at different points, the distance of these from 
the earth's centre, and, consequently, the figure of the earth, are 
determined. By this means the figure of the earth is found to 
be not a perfect sphere, but slightly flattened at the poles, so as 
to make its polar about twenty-six miles less than its equato- 
rial diameter. 

As the pendulum, in order to vibrate seconds in any place, 
must be always of the same length, it serves as an invariable 
standard of linear and cubic measures, and has been proposed as 
the universal unit of measure.* 

* The unit of linear measure is the yard, which is 1.086158 of the second's 
pendulum. The unit of measures of weight is the avoirdupois pound, e'^'sr 
of a cubic foot of pure water at its maximum density. In the United States, 
however, the pound troy (5762.38 grains) is the standard weight ; and the 

Uses of the pendulum ? How may it be used in determining the figure of the 
earth? 

6* 



54 



THE PENDULUM. 



43. By far the most useful application of the pendulum is to 
clocks as a measure of time. A pendulum vibrating alone, 
independent of any mechanism, would measure the time which 
elapses during its oscillation ; and to ascertain this would require 
only that an observer sit by and count the number of its oscilla- 
tions. If the time of one oscillation were 
previously known, then the number of these 
performed in any interval would at once give 
the length of such interval. But, in order 
to supersede the attention and vigilance of 
such an observer, a train of wheel-work is 
placed in connection with the pendulum, the 
movement of which it regulates ; and in con- 
nection with this wheel-work are fixed the 
dial-plate and the hands of the clock, by 
which the number of vibrations or oscillations 
of the pendulum which take place in a day, 
or in any part of a day, is indicated and reg- 
^W istered.* 

Fig. 29 shows the manner in which the 
pendulum regulates the movements of the 
clock. A toothed wheel is fixed upon an 
axis, around which winds a cord ; to the end 
of this cord is attached a weight, W. Were 
there nothing to intercept, this weight would 
fall with an accelerated velocity, causing a 
proportionate revolution of the wheel; its 
progress, as that of the wheel, is, however, 
arrested by the pallets, P P', attached to the 

Wincliester bushel (2150.4 cubic inches, or 77.6274 pounds of pure water) the 
standard for dry vieaaure, while the English wine gallon (231 cubic inches, or 
8.339 avoirdupois pure water) is the standard for liquid measure. 
* Lardner. 



The most useful application of the pendulum ? State how this is applied as 
a measure of time. Explain the figure. 



FRICTION. 55 

axis around which the pendulum vibrates. Thus, when the pen- 
dulum is in the position seen in the figure, the revolution of the 
wheel is arrested by the pallet, P'. As the pendulum swings 
back, P' rises, and allows the wheel to move one tooth, when it 
is again arrested by the pallet, P, which descends and meets the 
tooth beneath it. Thus, with each oscillation of the pendulum 
a tooth escapes, and hence the term escapement applied to this 
contrivance. In this way a continued motion is given to the 
pendulum, which in turn regulates the movements of the clock. 
By interposing a sufiicient number of wheels between the pen- 
dulum and the weight, clocks are made to run a month, or even 
a year, without winding. 

44. Friction offers the chief resistance to moving bodies. 
This is of two kinds, sliding and rolling. When a heavy body 
having a polished surface is made to slide over another polished 
surface, the friction between the two surfaces is considerable. 
This is due to the minute irregularities on these surfaces. Oil, 
tallow, or plumbago, applied, serves to fill up and smooth these 
irregularities, and thus diminish friction. Friction produced by 
rolling bodies is far less than that produced from sliding bodies 
of equal weight ; thus the same weight supported on wheels is 
moved with far less force than when resting on a drag. The 
friction of a machine is commonly estimated as equal to one third 
its power. 



PRACTICAL PROBLEMS IN MECHANICS. 

1. In a lever of the^r^^ kind, 6 feet in length, the power is 75 
and the weight 150 lbs. ; where must the fulcrum be placed that 
these may balance ? {^ 24.) 

2. If a lever of the/r^z^ kind, 8 feet long, have its fulcrum 2 feet 

How does friction affect moving bodies ? Kinds of friction ? Explain the 
effects of friction and causes of these effects. How may this be overcome ? 
"What portion of the power of a machine is estimated as destroyed by friction? 



5Q PROBLEMS. 

from the weight at one end, and this weight be 450 lbs., what power 
at the other end of the lever will balance ? 

3. A lever of the second kind is 20 feet long ; at what distance 
from the fulcrum must a weight of 80 lbs. be placed, so that it may 
be sustained by a power of 60 lbs. ? 

4. From a pole 8 feet long, resting on the shoulders of two men, 
is suspended a weight of 220 lbs., the point of suspension being 3 
feet from the first, and 2 feet from the second man ; what weight will 
each sustain ? 

5. In a lever of the third kind, 6 feet long, if a power of 150 
lbs. be applied 2 feet from the fulcrum, what weight will it raise at 
the other end of the lever ? 

6. In the compound lever. Fig. 20, what weight at D will a 
power of 75 lbs. applied at A raise ? 

7. A power of 60 lbs. acts on a wheel 8 feet in diameter ; what 
weight suspended from a rope winding round an axle 10 inches in 
diameter will balance this power ? 

8. In a system of pulleys shown at K, Fig. 22, what weight will 
a power of 100 lbs. sustain ? 

9. In the system shown at M, Fig. 22, what weight will a power 
of 50 lbs. sustain ? 

10. If a man has just strength sufficient to lift a barrel of flour, 
weighing 196 lbs., perpendicularly, so as to load it into a wagon 3 
feet high, what weight could he raise by means of a plank, with one 
end resting upon the wagon, and the other on the ground 10 feet 
from this? 

[In this case the power (196 lbs.) is to the weight as the height 
(3 feet) of the plane is to its length (10 feet).] 

11. With what force will a weight of 1,200 lbs. pi'ess on an in- 
clined plane, the length of which is 40 feet and the base 25 feet ? 

[The weight is to the pressure upon the plane as the length of the 
plane is to its base.] 

12. Suppose a power of 60 lbs. be applied at the end of a lever 
4 feet long, attached to a screw, the distance between the threads of 
which is ^ of an inch ; what weight will such a power sustain ? 
(^ 33.) 

13. If the length of a pendulum to vibrate seconds at Boston 
be 39,101 inches, how long must it be to vibrate half seconds? 
(^ 41.) 



HYDROSTATIC PARADOX. 57 



HYDROSTATICS. 



45. Hydrostatics is that branch of natural philosophy 
which treats of the mechanical properties of liquids. 

Liquids when at rest transmit their pressiire equally in 
all directions. — It is this remarkable property which partic- 
ularly distinguishes liquids from solids ; for, while the latter 
press only downwards in the direction of gravity, liquids press 
in all directions, — downwards, upwards, and sideways. Thus, 
if the downward pressure of a liquid confined in a vessel be 
known for any depth below its surface, experiment shows its 
lateral and upward forces at this depth to be the same. 

It is on this principle that the hydrostatic jmradox is 
founded, which consists in the fact of a small column of water 
balancing a larger. Thus, in a teapot filled with liquid, the 
small column of this in the neck balances the larger in the 
body of the vessel, causing both to stand at the same level. 

46. If a quantity of liquid be confined in a vessel, a mechan- 
ical force exerted on any portion of it will be at once trans- 
mitted through the whole mass. Thus, if a large and tight 
cistern filled with water have two small holes through remote 
parts of its top, upon forcing a cork into one of these, the 
pressure exerted upon the liquid directly beneath this will be 
instantly transmitted through the whole mass and felt at the 
other opening.* 

* Owing to the speed and facility with which liquids transmit a pressure 
upon them, tubes filled with water have been in some instances employed for 

transmitting signals between places separated by a distance of several miles. 

• 

Define Hydrostatics. State the proposition in regard to liquids at rest. 
On what principle is the hydrostatic paradox founded ? Illustration ? How 
is the pressure upon any portion of a mass of liquid transmitted? Illus- 
tration ? 



58 



HYDROSTATIC PRESS. 




The Hydrostatic Press acts on this principle. This won- 
derful machine may be illustrated by 
PTHI Fig. 30. In a small cylinder, A B, 
J moves the piston of a forcing-pump ; 
this connects, through a tube leading 
from its side at B, with a much larger 
cylinder, CD. In this moves also a 
piston, having the upper end of its rod 
at E pressed against a movable plank 
or iron plate, surrounded by a strong 
framework ; between this plank and the 
beam above is placed the substance to be pressed. By the 
action of the pump-handle water is raised into the cylinder. A, 
and on depressing the piston it is forced out through a valve 
at B and a pipe into the larger cylinder, C D, where it acts to 
raise the larger piston, and causes it to exert its whole force 
upon the objects confined between the planks or plates of the 
press. 

Now, as liquids transmit the pressure upon any portion of 
them in all directions, it follows that the pressure upon the 
piston at B will be transmitted to the piston in C D, increased 
in proportion as the area of the bottom of this exceeds the 
area of that at B. 

Thus the power of such a machine becomes surprisingly 
great; for, suppose the area of the end of the larger to be 
one hundred times that of the smaller piston, then, if, by means 
of the lever-handle, a pressure of one hundred pounds be 
exerted upon the smaller piston, it will transmit this pressure 
to every equal area upon the bottom of the larger, causing this 
to exert a force at E of ten thousand pounds. By this 
machine, the force of a child exerted upon the lever-handle 
may be* sufficient to crush the most stubborn objects. 

Explain the action of the Hydrostatic Press. What is said of the force of 
this? Illustrate the ratio of the power to the resistance by the area of the 
two pistons. 



HYDROSTATIC PRESS. 



59 



Figure 31 exhibits a sectional view of another form of tho 
Hydrostatic Press for measuring the amount of the force 



Fig. 31. 




exerted. Here the liquid is represented as pumped from a 
vessel through a tube attached to the bottom of the smaller 
cylinder, and forced through a drop- valve in the bottom of the 
larger. A small pipe leading from the bottom of the larger 
cylinder is provided with a stop-cock, to which a hose may be 
attached and a stream of water thrown to a great distance by 
hydrostatic pressure. The end of a lever, placed beneath a 
bracket fastened to the wall, may rest upon the upper extremity 
of the piston-rod as a fulcrum. In this manner the pressure 
which the larger piston sustains may be determined. 

47. The action of the Hydrostatic Press is based upon the 
principle that opposing forces are in equilibrium when their 
momenta are equal. Thus the momentum of the smaller piston 
may be regarded as the product of the space through which it 
moves into the area of its bottom : and the same also in regard 
to the larger. If the area of the larger piston be one hundred 
times that of the smaller, and the latter descend one inch, the 



What does Fig. 31 illustrate ? On what principle is the action of the Hy- 
drostatic Press based ? Illustrate this. 



60 LEVEL OF LIQUIDS. 

larger piston will be raised only y^^ of an inch. Thus, a small 
power upon the smaller piston may be made to balance a great 
weight upon the larger, by making the space moved over by the 
former as much greater than that moved over by the latter 
as the resistance or weight upon the larger exceeds the force 
exerted on the smaller piston. 

48. The surface of a liquid when at rest is level. — The 
property in liquids of maintaining a horizontal surface is due 
to the slight cohesion among their particles, which allows them 
to yield to the tendency of matter to gravitate towards the 
earth's centre. Mountains and hills would flow down, and the 
whole surface of the globe become uniformly level, were it not 
for the cohesion among their particles, which is superior to their 
gravitating force. 

This tendency of liquids to maintain a level, under all cir- 
cumstances, may be illustrated by an arrangement seen in 
Fig. 32. JFig. 32. Several 

glass vessels of differ- 
ent shapes are fixed 
in a stand, and con- 
nected by a pipe 
leading between their 
bottoms. If now water be poured into either of these, it will 
flow along the pipe, and be found to rise to the same height or 
level in each of the other vessels. Hence it is that aqueducts 
may be made to convey water over uneven surfaces, as hills 
and valleys, provided the point of delivery be not higher than 
the source from whence it flows. The play of the property in 
virtue of which liquids maintain their level, explains an infinite 
variety of important and interesting phenomena attending the 
circulation of water on the surface of the globe. By the nat- 



State the proposition in regard to the surface of a liquid at rest. Cause of 
this level ? What does Fig. 32 illustrate ? Why is it that aqueducts may be 
made to convey water over uneven surfaces ? 




HYDROSTATIC BELLOWS. 



61 



I 



T 



ural process of evaporation, the clouds become charged -with 
vapor, and are attracted by the lofty ridges of mountains, and 
all other elevated parts of the land around which they collect, 
and upon which they discharge their contents. 

The water thus deposited upon the highest parts of the 
globe has a constant tendency to return to the general level of 
the sea, and, in finding its way thither, gives rise to the phe- 
nomena of streams, rivers, cataracts, lakes, springs, and all the 
infinite variety of effects attending the movement of water wit- 
nessed over the earth's surface. If the 
waters which fall from the clouds encoun- 
ter a soil not easily penetrable, they 
^^^^ collect in rills, and form streams and 

^SF i rivulets, and descend along the sides of 

the elevation, seeking constantly a lower 

level. These encounter in their course 
other streams, with which they unite, until 
they at length swell into a river, winding 
and widening'in its course until its waters 
are again restored to the ocean, from 
whence they were taken. Throughout the 
whole of these phenomena the principle 
in operation is the tendency of liquids to 
maintain their level. ^ 
I 49. The pressure exerted by a col- 

wnn of liquid is as its height, and not 
as its quantity. — This proposition may 
be illustrated by the Hydrostatic Bel- 
lows, Fig. 33. This instrument consists 
of two boards united by a flexible leather 
or cloth like a common bellows. A ver- 

* Lardner. 



^/*.i5-Oa__, 




Explain the phenomena of the formation and flow of streams on the earth's 
surface. State proposition in section 49. By what instrument may this be 
illustrated ? Describe the Hydrostatic Bellows and manner of its use. 



62 



HYDROSTATIC BELLOWS. 



tical tube attached to the side communicates with the interior 
space. ' 

Experiment . — The bellows, when empty, may be loaded 
with weights. Water poured into the tube will be found to 
raise the upper board and weights, and, as the height of the 
vertical column in the tube is increased, so, in like proportion, 
may the weights upon the bellows be increased and supported.* 
In this experiment it matters not in regard to the size of the 
tube ; the column in the forms represented in the figure, will 
exert the same pressure at the same height, f The upward 
pressure upon the upper board of the bellows, will be equal to the 
weight of a column of water resting on an area equal to this 
board, and of the same vertical height of the liquid in the tube. 
Fig. 34 exhibits another form 
of the same instrument, where the 
upper board is divided into one 
hundred squares, each having the 
same sectional area as the square 
' tube containing the vertical column. 
Thus the proportional areas of the 
columns of the tube and bellows 
are definitely shown. At B is a 
a three-way stop-cock, by which the 
a communication between the tube 
and bellows is cut off; the water may be drawn from the bel- 
lows, and not from the tube, or from the tube, and not from the 
bellows ; or it may be closed to either, while the tube is re- 
moved and another introduced. 



Fig. 34. 




* A small quantity of water should be poured into the bellows, suflBcient to 
separate the boards a trifle, before placing the weights upon these. 

t Different forms and sizes of tubes, with their funnels, are represented in 
the figure. These tubes are made to screw together, so as to increase the 
height of the column. 



To what is the upward pressure upon the upper board equal ? 



HYDROSTATIC PARADOX. 



63 



Experiment a. — That the pressure of a liquid is as its height 
may be illustrated by inserting a long tube in the end of a 
strong cask, and then pouring in water. As the cask becomes 
filled, and the liquid rises ui the tube, a pressure will be exerted 
proportional to its height. In this manner the column of water 
contained in a small tube may be made to burst the strongest 
cask. 

From the same causCj rocks are sometimes cleft asunder, and 
the sides of mountains forced off; water penetrating the earth 
to great depths, and filling cavities in these, and then rising in 
the entrance so as to exert a hydrostatic pressure equal to these 
results. 

50. The pressure upon the bottom of a vessel^ containing 
a liquid^ is not affected by the shape of the vessel^ but is as 
the depth below the surface of the liquid. Thus, w^hether the 
vessel have its sides diverging, converging or perpendicular, 
the pressure upon a bottom of the same area and depth will be 
equal. 

This may be experimentally verified by the apparatus seen in 



rig. 35. 



iri 




Fig. 35. Here a brass tube, mounted on a stand, has its lower 
extremity entering a basin of water ; upon the top is screwed a 



How may the same proportion be illustrated by means of a strong cask and 
tube ? Other illustrations ? State the proposition in section 50. Explain the 
illustration of this proposition by the figure. 



64 



PRESSURE OF LIQUIDS. 



broad glass funnel ; a plunger, nicely fitted to the brass tube, is 
attached by a small chain to the end of a scale-beam. When 
the funnel is filled with water to a given height, — say seven 
inches, — balance the scale-beam by weights placed in the op- 
posite scale-pan ; the pressure of the liquid upon the plunger, 
with this arrangement, may be thus ascertained. Now remove 
the funnel, and substitute the glass tube A, of the same sectional 
area as the brass one ; fill this with liquid to the height of seven 
inches, and ascertain the pressure sustained by the plunger as 
before ; remove this, and in its place substitute the small tall 
tube D, and fill to the same height, and ascertain the pressure 
upon the plunger, as in the two previous instances. In each in- 
stance the pressure of the liquid upon the plunger will be found 
the same ; proving this pressure to be not afiected by the shape 
of the vessel, but as the depth below the surface. 

51. Since the pressure of liquids increases with the depth, a 
proper regard should be had for this in the construction of 
dams and embankments for confining water, causing these 
gradually to increase in thickness and strength from the top 
towards the bottom.* 

Striking illustrations of the increase of pressure, in de- 
scending below the surface of liquids, are furnished by sinking 
bodies in the ocean. Thus, if an empty bottle, tightly corked, 



* The following table 
depths : 



the pressure of water in pounds, at various 



Dei)th in feet. 


Pressure per square 


nch. 


Pre 


ssure per square foot. 




lbs. 






lbs. 


1 


0.432 






62 323 


2 


0.865 






124.646 


3 


1.298 






186.969 


4 


1.731 






249.292 


5 


2.164 






311.616 


6 


2.596 






373.939 


7 


3.029 






436.262 


8 


3.462 






498.585 


9 


3.895 






560.908 


10 


4.328 






623.232 



Where should this pressure of liquids be regarded ? 



NOX-COMPRESSIBILITY OF WATER. 65 

be sunk, bj Tveights attached, to a certain depth, — saj five or 
six hundred feet, — it will be either crushed or the cork forced 
in ; showing the enormous pressure to which it is subjected at 
that depth. 

If a piece of wood, which floats on the surface of the water, 
be sunk in the same manner, the liquid will be forced into its 
pores bj the surrounding pressure, so that it will be unable 
again to rise to the surface. 

In plunging below a certain depth, divers often find the pres- 
sure so great as to rupture the more delicate vessels of the body, 
and do serious injury. It is said that the Greenland whale 
sometimes descends to the depth of a mile, but always comes up 
Fig. 36. exhausted and spouting blood : showing that the pressure 
' had so acted on the vessels as to cause them to dis- 
charge a portion of their contents into the lungs. 

52. Whether water be compressible or not is a dis- 
puted question among philosophers. The following ex- 
periments, performed before the secretary and regents 
of the Smithsonian Institute, in 1849, go far towards 
substantiating the idea that it is non-compressible. 
Fig. 36 gives a partial view of the apparatus em- 
ployed ; and the manner of using this may be learned 
from the following 
Experiment. — Fill with water the small bottle, to the 
tubular neck of which is affixed a minute scale. Place in the 
small cup at the top of this neck, a globule of mercury ; lower 
this small bottle thus prepared into the strong glass cylinder ; 
beside it may be placed a small condensing -guage for showing 
the degree of the compression. Fill the cylinder with water, 
and enter the screw, which works air-tight ^ through the thick 
brass cap. The water in the cylinder will undergo a powerful 

Give illustrations of the pressure exerted by liquids in case of a tight bottle 
sunk in the ocean. In case of wood. Results of diving below a certain 
depth ? Case of the whale ? What is said of the question in regard to the 
compressibility of water ? 
6* 




SPECIFIC GRAVITY. 



Fig. 37. 



compression, which will act on that in the bottle, causing it to 
occupy less space, as denoted by the sinking of the mercury in 
the tube or neck. This might be considered a satisfactory 
proof of the compressibility of the water in the bottle, and 
hence of liquids generally, were the experiments to end: here. 
Experiment a. — Now remove the small bottle from the cyl- 
inder, Fig. 36 ; invert and suspend it from a 
sliding-rod under the receiver of an air-pump, 
Fig. 44, in a manner similar to that seen in 
Fig. 37 ; draw up the sliding-rod so that the 
mouth of the bottle shall just clear the wa- 
ter in the jar. Exhaust thoroughly, so as to 
free the water, as far as possible, of its air ; 
then let the neck into the water, and admit 
the air slowly into the receiver again. The 
bottle is now filled with w^ater, nearly freed 
from air. Invert and place a globule of mer- 
cury in the cup, as before, and arrange the 
whole in the glass cylinder, as in the last ex- 
periment. Work down the screw-plug, and 
the water in the small bottle now undergoes 
no perceptible diminution of its volume ; 
proving that in the celebrated experiment by 
Professor Oersted, so generally copied, it is the air in the 
water, and not the water itself, which undergoes compression. 




SPECIFIC GRAVITY. 

53. If a solid body be accurately weighed in air, and then 
immersed in a vessel filled to the brim with water, and weighed 
again, it will be found to have lost in freight, or to be buoyed 
up by a force exactly equal to the weight of the water displaced 
by the solid, and which has flowed over the sides of the vessel. 



Result of experiments ? What is said of a solid body weighed in air and 
then in water ? What is this difference in the weight of the body equal ? 



SPECIFIC GRAVITY. 



67 



The difference, therefore, between the weight of the body in air 
and in water, will be the weight of a quantity of water of the 
same bulk as the solid body. If this quantity be heavier than 
the solid, it will float ; but, if lighter, the solid will sink. 

The specific gravity of a body is its weight compared with 
the weight of an equal bidk of some fluid. Pure water is 
usually taken as the standard for solids, and air for gases. If 
the weight of water be taken as unity, the weight of an equal 
bulk of a solid, heavier or lighter than this, will be expressed 
by a number greater or less than unity. 

54. To find the specific gravity of a solid body heavier 
than water. — Ascertain its weight in air, and then again in 
water, and divide its former weight by the difference between 
the two weights ; the quotient will express the specific gravity 
of the solid. 

To find the specific gravity of a body lighter than water. 
— Tie to it any heavy solid, whose weight in air and water 
is known, and sink the whole in w^ter. Weigh the compound 
both in air and water, and ascertain the loss of weight; 
then, knowing the weight lost by weighing the heavy body by 
itself in water, ascertain the difference of these losses, and divide 
the weight of the lighter body by this difference ; the quotient 
will be its specific gravity. If the body be 
soluble in water, it may be covered with a 
coating of varnish, or be weighed in some 
other liquid whose specific gravity in relation 
to water is known, and which will not dissolve 
the solid. Fig. 38 exhibits a convenient ar- 
rangement of the scales, and the maimer of 
suspending the body in water for finding its 
specific gravity. The body should be, if pos- 

What do you mean by the specific gravity of a body ? What is taken as the 
standard for solids ? For gases ? What rule for finding the specific gravity 
of a solid heavier than water ? Kule for one lighter than water ? What does 
Fig. 37 show ? How should the body be suspended ? 




bo HYDROMETER. 

sible, suspended bj a horse-hair, or fine waxed thread, "where 
nice calculations are required.* 

55. To deterirnine the sj^fecijic gravity of a liquid. — As- 
certain the weight of a given quantity of pure water in a small 
vial, and then the weight of an equal quantity of the liquid in 
the same vial ; divide the weight of the liquid by that of the 
water, and the quotient will be the specific gravity of the liquid. 

There are various other modes of determining the specific 
weights of solids and liquids. We have only given such as are 
more simple and easy of performance. 

The Hydrometer^ is an instrument used in commerce for de- 
termining at once the specific gravity of liquors, such as alcohol, 
acids, etc., and thereby the degree of their purity. The indi- 
cations of this instrument all depend on the fact that a solid 
body, when it floats in a liquid, displaces a quantity of this 
equal to its own weight ; consequently, if the liquid be heavier 
or lighter, bulk for bulk, than pure water, the hydrometer will 
sink or rise proportionally below or above the point at which it 
stands in the latter, f 

* Table showing the specific weights of certain solids at their greatest den- 
sity. 

Platinum, 22.06 

Gold, 19.36 

Copper, 8.87 

Iron, 7.78 

Tin, 7.29 

Flint glass, 3.37 

Marble, 2.83 

Rock crystal, 2.68 

Potassium, 86 

t Table showing the specific gravity of certain liquids at their greatest den- 
sity. 

Distilled water, 1.00 

Mercury, 13.59 

Concentrated sulphuric acid, 1.84 

How may the specific gravity of a liquid be found ? What is the Hydrom- 
eter ? On what principle does it act ? 




FLOATING BODIES. by 

Fig. 39 exhibits a common form of the hydrometer. This 
may be of glass or metal, and consists of two bulbs, to the lar- 
ger and upper of which is attached a small stem, graduated fiom 
the top downwards. In the lower and smaller bulb 
is placed some mercury, or shot, sufficient to sink 
the bulb to a certain depth, and cause the stem to 
maintain a vertical position. The point at which 
the instrument stands in pure water, at its greatest 
density, is marked ; that at which it 
stands in pure alcohol, 100. The adul- '^'^' ^°* 
teration of alcohol and other liquids 
lighter than water will add to their weight, 
which will be at once shown by the less 
depth to which the hydrometer will sink in 
these. For determining the specific grav- 
ities of acids and liquids heavier than water, S 
a different graduation is required, and a 
small circular lead weight is slipped on the 
neck between the bulbs. Such is shown by Fig. 40- 
56. Bodies float on water agreeably with principles just con- 
sidered. Thus, a ship laden with a heavy cargo is buoyed up 
from the fact that the weight of the water it displaces equals that 
of the entire ship and cargo.* So a tin pan or an iron boat floats, 

Fuming nitric acid, 1.45 

Concentrated hydrochloric acid, 1.20 

Pure alcohol, 79 

Ether, 71 

Sea-water, .^ 1.02 

Milk, 1.03 

Naphtha, 84 

* By knowinor the weight of a given bulk of water (a cubic foot), and then 
having the cubical dimensions of a vessel given, its tonnage may be readily de- 
termined. This may be found by subtracting the weight of the water dis- 

Construction of the hydrometer? How used? Why do bodies float on 
liquids ? 



70 MOTION OP LIQUIDS. 

although made of materials whose specij&c gravities far exceed 
that of water, since thej are so formed as to displace a weight 
of water equal to that of their own weight. 

The human body, when the lungs are filled with air, is a 
trifle lighter than the same bulk of water, and, consequently, 
floats on this. In breathing out the air from the lungs it 
becomes heavier, and so to maintain itself on the surface requires 
a slight efibrt with the hands and feet. The bodies of some 
persons are lighter, bulk for bulk, than others ; such are more 
buoyed up, and consequently swim with less efibrt. The bodies 
of drowned persons, after being beneath the surface a certain 
time, rise and float, owing to the inflation of the body by gases 
generated in decomposition. % 

LIQUIDS IN MOTION. 

57. The branch of Hydrostatics which considers the flow of 
liquids through orifices in the sides of vessels, through pipes, in 
rivers, canals, etc., and their efiects on solid bodies moving in 
them, is termed Hydraulics. 

The various contingent causes which act to modify the flow 
of liquids render it not easy to bring this class of phenomena 
within fixed and prescribed rules. The analogy, however, 
between the efiects of liquids and solids in motion, may be traced 
to a certain extent. 

placed ■when the vessel is unladen, from the weight it Tvill displace when 
loaded down to a certain point ; the diflference will give the weight of the 
cargo or tonnage it will carry. Camels, used in taking loaded ships over 
sand-bars, consist of tight and strong wooden boxes or tanks of large dimen- 
sions, attached while empty to the sides of the ship ; the buoyancy of these 
prevents the vessel from drawing as much water or sinking as deep as it 
otherwise would. 

State the case in regard to the human body. Why are some persons buoyed 
up more than others ? Why do the bodies of drowned persons after a certain 
time rise to the surface ? Define Hydraulics. What is said in regard to the 
flow of liquids ? 



RESISTANCE OF LIQUIDS. 71 

The square of the velocity of a liqiiid esca'piiig from an 
opening in the side of a vessel is as the depth below the sur- 
face. — This may be inferred from the fact already stated 
{§ 49), that the pressure of a liquid is as its depth. If several 
openings be made in the side of a vessel filled with water at the 
depth of one, four, nine, and sixteen feet, the velocities with 
which the liquid will escape from these will be in the propor- 
tion of one, two. three, and four. Thus, by knowing the 
velocity at any given depth, the velocity of the liquid at any 
other depth may be readily determined. 

58. A liquid escaping from a reservoir, through a jet, open- 
ing upwards, would rise as high as the surface of the liquid in 
the reservoir, provided it encountered no friction or resistance 
of the air. The pressure of a liquid at any depth corresponds 
to the velocity of a falling solid at that depth ; now, since a 
solid body acquires in falling from a given height a velocity 
sufiicient to carry it up again to that height, so will a liquid 
issuing vertically upwards from a jet rise as high as the surface 
of the liquid in the reservoir, provided it be not obstructed by 
the causes just mentioned. 

The rate of the flow of a liquid from a reservoir is greater 
through a short pipe, whose length bears a certain ratio to the 
orifice, than through the orifice alone. 

59. The resistance offered to a solid body moviiig through 
a liquid varies with the form of the solid. — If the surface 
presented to the liquid by a body moved perpendicularly against 
it, be flat, the resistance will vary with the magnitude of the 
surface. If, instead of being presented perpendicularly to the 
liquid, the surface be presented obliquely with respect to the 
direction of its motion, the resistance will be diminished on two 
accounts ; first, the quantity of liquid displaced will be less, 
and, second, the action of the surface in displacing it will have 

State the proposition in regard to the flow of a liquid from openings in the 
side of a vessel. Illustrate this. State the proposition in section 58. Illus- 
trate this. 



72 



WATER-WHEELS. 




the mechanical advantage of an inclined plane or wedge, so that, 
instead of driving the liquid forward, it will, in some measure, 
push it aside. 

The success in naval architecture depends upon a proper 
regard to these principles.* 

60. The force of water in motion renders it highly service- 
able as a motive power, and accordingly it is extensively applied 
in mechanical and manufacturing operations. Its application is 
usually to wheels, against which it exerts its 
force in various ways. The three common 
forms of water-wheels are the over- shot ^ the 
under-shot^ and the hreast-wheel. In the 
first, the water exerts its force upon the upper 
descending side of the wheel ; in the second, 
upon the floats against which it flows upon 
the under side ; in the breast- wheel, the water 
falls on a point nearly in a line with the axis, 
and acts chiefly by its weight. 

A novel form of acquiring water-power 
has been, in some instances, resorted to by a 
contrivance known as Barker's Mill, where 
water is made to flow down a tube, and out 
through openings made on opposite sides 
near the extremities of two horizontal tubular 
arms. The unequal pressure exerted on the 
sides of these arms near their extremities by 
these discharges of the liquid causes them to 
revolve and turn a vertical shaft moving in the ' 
vertical tube, and to the upper end of which 
the machinery is attached. The principle of Barker's Mill 

* Lardner. 




On what does the success of naval architecture depend ? What is said of 
the force of water in motion ? How is it usually applied ? The kinds of water- 
wheels ? How does the water exert its force on them ? What is said of Bar- 
ker's Mill ? Its construction and manner of operating ? 



AKCHIMEDES' SCREWY. 



may be illustrated by a simple arrangement, seen in Fig. 41, 
vrliere a revolving jet is attached to the lower extremity of a 
small tube provided with a funnel, and into which water is 
poured. The flow of water in opposite directions from the 
jet will cause it to revolve rapidly. 

61. Archimedes' Screiv is a hydraulic instrument invented 
by Archimedes, for draining certain portions of the valley of the 
Kile after the overflowings of the river. It was also used by the 
ancients, before the discovery of pumps, as a means of clearing 
the holds of vessels from water. 

This machine is represented in Fig. 42, and consists of a 
tube wound in a spiral form about a cylinder. This cylinder is 
placed at a certain inclination, with its lower extremity resting 

in the water. As 
the cjdinderis made 
to revolve, the end 
of the tube dips in- 
to water, and takes 
up within it a por- 
tion of this ; this 
water continually 
flows to the under 
side of the tube, 
and, if the cylinder 
have not an angle with the horizon too great, the liquid will be 
raised and discharged out through the upper end of the tube. 




PRACTICAL PROBLEMS IN HYDROSTATICS. 

1. What pressure will a bottle, with a superficial area of 1 square 
foot, sustain when lowered into the sea to the depth of 500 feet ? 
[The pressure will be equal to the weight of 500 cubic feet of water. 
See § 51.] 



TVhat is said of the Archimedes' Screw ? Explain its construction and man- 
ner of operating. 

7 



74 PROBLEMS. 

2. If a cubical box, each side of which contains 9 square feet, 
be filled with water, and then a tube, inserted in its top, be also 
filled to the height of 25 feet, what pressure will be exerted on each 
side by the water in the tube alone, and what on the whole interior 
surface of the box ? 

3. What pressure will be sustained by a plank, 14 feet long and 
16 inches wide, placed along the bottom of a floom 8 feet below the 
surface of the water ? 

4. What weight of water will a hollow sphere contain, the inter- 
nal diameter of which is 10 inches, allowing a cubic inch of water to 
weigh .54 of an ounce troy ? 

5. If a stone weigh 12 lbs. in air, and 8.58 lbs. in water, what 
is its specific gravity ? (§ 54.) 

6. What is the specific gravity of a piece of ebony which weighs 
14 lbs. in air and 8 lbs. in water ? 

7. A piece of iron weighed 18 J ounces in air, and 16 ounces in 
a liquid ; what was the specific gravity of the liquid ? (§ 53.) 

8. What is the weight of a block of granite 10 feet long, 3 feet 
wide, and 2 feet thick, the specific gravity of granite being 2.75, and 
a cubic foot of water weighing 1000 ounces avoirdupois ? 

9. What is the tonnage of a vessel which, upon receiving a full 
freight, displaces 2,420 cubic feet of water more than when empty ? 

10. What weight of mercury will an iron bottle holding 850 
cubic inches contain ? 

11. What pressure will the body of a pearl-diver sustain at a depth 
of 60 feet below the surface of the water, supposing the body to have 
a superficial area of 6 square feet, and the weight of a cubic foot of 
water to be 62i lbs. ? 

12. Fishes have been drawn from the ocean at a depth of 2,800 
feet ; at this depth, what pressure would one, having a superficial 
area of 1 square foot, sustain ? 



GASEOUS BODIES. 



75 



INTRODUCTION TO PNEUMATICS, AND DESCRIP- 
TION OF INSTRUMENTS * 

62. That class of fluids which are produced from solid and 
liquid bodies by the agency of heat are termed vapors. Such 

Fig. 43. 




AMERICAN AIR-PUMP. 

maintain their elastic form only so long as they are subjected 
* If desired by the instructor, this introduction may be omitted. 



^ O AIR-PUMP. 

to the influence of heat in various degrees. Steam, for exam- 
ple, which is produced from boiling water, is readily condensed, 
and made to resume its liquid state upon the removal of the 
heat. Bodies, however, which maintain their elasticity at all 
temperatures; are said to be permanently elastic. Such are 
known as Gases. 

Of elastic fluids Atmosi^heric Air afibrds one of the best 
examples, being, unlike steam and many of the gases, perma- 
nently elastic. This may, therefore, be employed to illustrate 
the mechanical properties common to all elastic vapors. 

Air^ in the strict sense, is a compound of the two gases, 
oxygen and nitrogen, mechanically mixed in certain definite 
and unvarying proportions — the term atmosphere being applied 
tu the whole body of gaseous matter that surrounds the 
earth, including, besides air, watery vapor, and a variety of 
attenuated matter commingled. 

The varied and important relations of the atmosphere ta 
man's being and happiness render it especially deserving his 
careful study. By its vital energy he lives and moves, while its 
forces contribute in many w^ays to his convenience and comfort. 

For illustrating successfully its mechanical properties, instru- 
ments of great delicacy and perfection of operation are required. 
Some of the more important of these we shall here explain, 
describing briefly their construction, uses, and liabilities, and 
the requisites for their successful operation. No written 
description of a pneumatic apparatus can, however, supersede 
the necessity of practical illustrations by the instructor. 

63. The Air-Piimp. — This instrument is employed for re- 
moving the air from the various forms of receivers, and is by far 
the most important instrument for illustrating the mechanical 
properties of air. A defect in this, or a want of the requisite 

What is said of air as an elastic fluid ? "What is the composition of air ? 
What does the term atmosphere comprehend ? Why is the atmospliere deserv- 
ing of careful study ? What are necessary for illustrating the mechanical 
properties of air ? For what is the Air-Pump employed ? 



77 



skill on the part of the operator, will be often attended by a 
general failure, however perfect be the minor instruments of a 
pneumatic set. As elementary works upon natural philosophy 
seldom give any serviceable hints in regard to the liabilities of 
failure in the use of this, or practical directions how to remedy 
these when they occur, we have thought it advisable here to 
offer such as in our judgment may seem most needed. 




64. Description of the Air-Pump.^ — Fig. 44 shows a form 
of the American Lever Exhaust-Pump^ now generally used 

* If an air-pump or diagram is at hand, the pupil may describe the parts 
of the instrument from this. 



What is said of its importance in pneumatic illustrations ? 
7* 



78 AIR-PUMP. 

in the larger scientific institutions of the country, and recom- 
mended by its simple construction and the rapidity and perfec- 
tion of its exhaustion. Within the barrel A, which stands upon 
the lower basement, moves a thin and nicely-packed piston. 
Upon the upper side of this piston is a small drop-valve^ which 
plays freely up and down on the piston-rod, closing over two 
holes, which admit the air up through the piston. Beneath a 
small dome-cap, ^, on the top of the barrel is placed a clapper- 
valve^ also opening up and closing over two holes.* This 
valve is of fine leather, and is confined and kept in place by 
the dome-cap, which rests down upon a thin projection from the 
side of the valve. The third and only additional valve is 
placed at the bottom of the pump-barrel, and serves only as 
a guaranty against leakage in case of a defect in either of the 
valves above. 

The piston-rod^ c, which connects with the pump-lever 
by means of the parallel rods^ a «, moves freely through an 
air-ti(jht packing, e, at the top of the barrel, and also through 
a guide upon the upper basement. By this arrangement, and 
the valve upon the top of the cylinder, the piston is made to 
move in a vacuum free from the pressure of the atmosphere, 
and thus its ease and operation are greatly facilitated. Midway 
in the horizontal connecting tube which leads from the bottom 
of the pump-barrel, is the vent-plug^ f. Here the air should 
always be admitted after exhaustion, and the oil poured in when 
necessary for lubricating the inner surface of the barrel. When 

* This, in the air-pumps recently constructed by Mr. Chamberlain, has been 
dispensed \Yith, and a small drop-valve^ so arranged as to preclude the possi- 
bility of failure in its operation, has been substituted. The dome-cap also 
closes over this valve by a single screw, and may be readily removed and 
replaced by the fingers. The whole is exceedingly simple and highly opera- 
tive. 

Describe the valves of the aii'-pump and their operation. What advan- 
tages from causing the piston to move in a tight barrel free from the atmos- 
pheric pressure ? 



AIR-PUMP. 79 

desired, this plug may be removed, and various attachments of 
pkt«s, hose, etc., made, as seen in Fig. 43. Upon the upper 
basement stands the pump-jjlate^ G, which is screwed to a hub 
attached to the extremity of the upright connecting tube. Upon 
this plate are placed the various receivers to be exhausted. The 
harometer-gaiicje^ A, employed for denoting the degree of the 
exhaustion, is a graduated glass tube, which is attached by a 
screw to the guard-box, i, just beneath the upper basement. 
To the lower end of this glass tube is screwed a small ^inerciiry- 
cistern^ 7, into which extends a steel tube tipped with platina, 
to prevent the entrance of air through the mercury into the 
barometer. A screw-plug^ k^ on the upper end of the guard- 
box, allows of various attachments of small gauges, etc., but 
should never he used for venting or oiling the pump. 

A larger and more perfect form of the air-pump is shown by 
Fig. 43, while a smaller, yet highly operative instrument is 
represented in Fig. 164. All these are similar in their plan of 
construction and operation, and, although differing, according to 
size, in the rapidity, yet are equally perfect in the degree, of 
their exhaustion.* 

65. Theory of the Operation of the Air-Pump. — The air- 
pump owes its power of exhausting to the expansive force of 

* Figure 45 shows an apparatus for determming the tightness of an air- 
Fig. 45. pump, and the degree to which it will exhaust. This con- 
CD sists of a small glass bolt-head, with its stem just entering 
f^^\ some colored liquid placed in a glass bottle beneath a receiver 
upon the pump-plate. As the pump is worked, the air will 
expand and flow out of the bolt-head, and upon again admit- 
ting it into the receiver, the liquid will rise and occupy the 
place of the removed air. If the exhaustion be good, a bubble 
of air no larger than a medium-sized shot will be found re- 
maining in the bolt-head. If the pump and receiver be abso- 
luiely tight, the liquid will not rise in the tube after an exhaustion, until the 
air is regularly admitted. This is one of the most rigid tests of the air-pump 
known. 

Point out the other parts of the air-pump. Explain the theory of exhaus- 
tion by means of the air-pump. 




80 AIR-PUMP. 

air. As the piston is raised from the bottom of the barrel, A, 
Fig. 44, it lifts the air above it, and forces it out through the 
drop-valve upon the top of the barrel ; a vacuum is thus 
formed in the barrel below the piston, which causes the air in 
the receiver upon the pump-plate to expand and flow into the 
barrel. As the piston descends, this air in the barrel passes up 
through the di'op-valve into the vacuum above caused by the 
descent of the piston. Upon raising the piston again, the air 
above it is condensed, and again forced out through the valve ; 
and so the operation goes on — the air in the receiver becoming 
more rare with each rise of the piston, until the expansive force 
of the slight portion remaining is too feeble to raise the valves. 
Thus it will be seen that it is impossible, by means of the air- 
pump, to produce a perfect vacuum, since a portion of air must 
remain in the receiver, in order to expel the remainder. 

6Q. D'u^ections for the use of theAir-Pump. — Work the 
lever with a firm and steady hand, so as to bring the piston en- 
tirely up and down at each stroke. Keep this and the rod oiled 
with a moderate supply of the best sperm oil. An excess of oil, 
especially of an inferior quality, serves to clog and prevent the 
perfect action of the valves. 

The piston should work free from atmospheric pressure ; con- 
sequently a leakage of the upper valve from any cause may be 
readily known by a pressure upon the piston, and a tendency of 
the lever to rise. In such a case remove the dome-cap, and 
cleanse or renew the valve. The drop-valve upon the piston 
may be examined by carefully removing, in a similar manner, 
the top or cap from the barrel. With proper care these valves 
seldom become inoperative. 

The glass receivers should be perfectly fitted to the pump- 
plate by a circular grinding with flour, emery, and oil ; if so, 
to insure tightness, the plate will only require to be wiped over 
with an oily rag, before placing on it the receiver, for an exper- 

Why cannot a perfect vacuum be formed with this instrument ? 



AIR-PUMP. 81 

iment. To guard against leakage, after a partial stroke of the 
lever, clasp the receiver with both hands, and give it a few circular 
turns of an inch or two back and forth, so as to crush out any 
particles of dust, and bring the two surfaces in immediate con- 
tact. Guard against scratching or marring in the least the 
surface of this plate. 

Extreme caution is necessary in the use of mercury, lest it 
be drawn into the tubes, and, by its corrosive action on the brass, 
ruin the pump. The inexperienced would do well to dispense 
with those experiments requiring the use of this and strong 
acids about the pump. Directions for the use of these will be 
given as the experiments arise. 

Should the platina he worn off the steel tube which enters 
the mercury-cistern, bubbles of air will be seen occasionally to 
enter the barometer, when the steel point will require to be 
tinned over or retipped with platina. The stop-cocks and screw- 
connections of a pneumatic apparatus should always be provided 
with oiled-leather washers. No washer is, however, required 
where the mercury-cistern screws upon the barometer, and the 
washer, where this attaches to the guard-box, should be but 
slightly oiled. 

Should a leakage accidentally occur about any joint, it may 
be temporarily stopped by applying a trifle of thick paint. 

Every article of a pneumatic apparatus should be kept free 
from dust and moisture ; and after using, especially with 
liquids, should be wiped dry and smeared over with an oily 
cloth. 

The taking to pieces and general repair of these air-pumps 
should never be attempted by those not familiar with their con- 
struction, for, although simple in form, yet they often baffle the 
skill of the amateur mechanic, unaccustomed to air-tight ]omi^. 
to bring together the different parts when once separated. One 
may be a very " curious"' mechanic, and able to dissect a man- 
ikin, clean a watch, or construct a puzzle-box, and yet not 
possess the requisite skill for filling a thermometer-tube, or 
adjusting the piston and valves of an air-pump. 



82 CONDENSER. 

67. The Condenser. — This, in its operation, is the reverse 
of the exhaust-pump, and is used for forcing air into vessels. 
The simple form is shown by Fig. 46, and consists of a 
straight brass barrel, with a plunger nicely packed 
Fig. 46. ^j^[j leather, and having a valve in the lower part 
^M^ opening downward, while another valve, opening the 
same way, is attached to the lower end of the screw- 
plug, which enters the bottom of the barrel. 

Theory of the operation of the Condenser or 
Force-Pump. — As the piston is drawn towards the 
top of the barrel, the air flows down through the 
valve into the space below ; upon forcing the piston 
down, the air in the barrel is prevented from escap- 
ing through the piston by the closing of its valve, 
and is accordingly forced through the second valve, 
upon the screw-plug, into the chamber or receiver. 
The elastic force of this air closes the valve, and thus 
^^r prevents its escape back into the barrel. As the 
piston rises, the barrel is again filled with air, which, 
upon the descent of the piston, is forced through the lower valve 
into the chamber, as before ; and so the process may be continued 
until the air in the chamber has acquired a density and an expan- 
sive force truly surprising, as will be shown in the subsequent 
experiments. 

These valves are simple in their construction, and when 
deranged may be easily repaired. By reversing the screw-plug, 
and also the parts of the piston which screw upon the end of the 
piston-rod, so as to bring the valves upon the upper side, this con- 
denser may be readily converted into an exhaust-pump. No 
water should be allowed to remain in the barrel, as it will 
serve to stiffen and injure the packing. When, from any cause, 
the piston has been withdrawn from the barrel, some skill may 
be requisite for entering it again. To enter the p'lston^ place 
the lower edge of the leather rim within the barrel ; with the 

Describe the simple form of the Condenser. Explain the theory of its oper- 
ation. 



CONDENSING-GAUGE. 



83 



Fig. 47. 




thumb and fore-finger press down the upper edge, and at the 
same time turn the piston, so as to enter by degrees^ taking 
care not to let the edges of the leather get turned up. 

68. The Doiihle- Acting Exhauster and Condenser^ Fig. 
47, is a convenient instrument for the 
transfer of gases, etc., by exhausting 
from one vessel, and condensing into 
another. The piston, which is packed 
to work both ways, has no valve. In 
the bottom of the barrel are two 
clapper-valves, one opening up, and 
the other down. The barrel turns on 
an even and nicely-fitted plate at the 
bottom, to which it is tightly pressed 
by means of the two small screws 
which enter the binding-ring ^.^^ ^g 
above. As arranged in the 
cut, it exhausts through the 
bottom hole, while it con- 
denses out at the side stop- 
cock. Now turn the barrel 
so as to bring the valves 
each over the other opening, 
and the whole operation will 
be reversed. 

69. The Condensing- 
Gaiige^ Fig. 48, is used in ""^"^"^ 
determining the degree of pressure to which air, steam and gas 
are subjected, and is screwed to the condensing-chamber, or 
other vessel into which these are to be forced. The pressure is 
indicated by the rise of the mercury in the sealed arm of the 
glass tube — the space of air being inversely as the pressure.* 




* Beware of mistaking this for the siphon-gauge below, and so connecting 
it with the exhaust-pump, as in such case an expensive mistake may occur by 
the mercui'y being drawn over into the tubes of the pump. 



84 



VACUUxM- GAUGE. 



49. 




The Siphon Vacuum-Gmige^ Fig. 49, may be used to 
show the degree of the exhaustion. The dark por- 
tion of the fine glass tube seen in the cut is filled 
-with mercury. As the 2:)ressure of the air which 
forces the fluid into this arm of the 
tube is removed by exhaustion, the 
mercury falls in one and rises in 
the other arm. coming nearer to a 
level according as the space ap- 
proaches a vacuum. This form of 
the gauge is used with the smaller 
air-pumps, and may be attached to 
the larger, at the screw over the 
guard-box. The strong glass cap, 
which covers this and the condensing-gauge, 
serves to protect them against breakage. 
The theory of these gauges will be explained 
in the subsequent experiments. 

TO. The Guinea and Feather Tiihe^ 
Fig. 50, used to illustrate the equal fall 
of light and heavy bodies in a vacuum, and 
also to furnish various electrical illumina- 
tions, is a long, brass-capped glass tube, used 
in connection with the air-pump. When 
screwed to the centre hole of the pump- 
plate, it should be with care, lest the stop- 
cock be wrenched off where it enters the 
hole. When the air is admitted after an 
exhaustion, the light bodies should be allowed to fall to the 
other end of the tube, lest they be injured by the rush of air 
through the stop-cock.* 

71. The following cuts present accurate views of some of thj 
more important articles employed for formincj connections be- 

* These tubes may be used for showing the effect of condensed air on falling 
bodies, and are made for sustaining a pr(.>.-.ure of four or five atmospheres 




SCREW CONNECTIONS. 



85 



tween the various parts of a pneumatic apparatus. To these we 
shall have frequent occasion to refer in the following pages. 



Fig. 51 



Fig. 53. 




Figures 51 and 52 are much used for attaching the condenser, 
hose, etc Fig. 53, small Stop- Cock. Fig. 54, large Stop- 



86 



SLIDING-RODS. — RUBBER HOSE. 



Cock. Fig. 55, Connecting- Screw. Fig. 5Q, Guard-Screw. 
Fig. 57, Screw-Plug. Fig. 58, Gallows- Connector Exterior 



Fig. 58. 





Fig. 61. 



Fig. 62. 



Fig. 63. 




Screw. Fig. 59, Gallows-Connector Interior Screw. Fig. 60, 
Plate and Sliding-Rod. Fig. 61, and Fig. 62, Sliding-Rods 
with Screw-Plugs and Packing. Fig. 63, Flexible Rubber 
Hose. 



AIR. 87 



PNEUMATICS. 

72. Pneumatics explains the laws which regulate the flow 
and equilibrium of elastic fluids. 

Fluids are bodies whose particles glide easily among them- 
selves, and tend readily to a level or equilibrium. These are 
divided into two classes, elastic and inelastic fluids. To the 
former belong the gases and vapors ; to the latter, water and 
other liquids. Elastic fluids are also comprehended under two 
divisions ; those like atmospheric air, many of the simple gases, 
which can never, by any kno\Yn agencies, be made to yield 
up their elasticity and assume a difierent form, and those like 
steam, carbonic acid, etc., whose elastic natures depend on the 
circumstances of heat and pressure. 

Air and steam are the elastic fluids chiefly employed as 
mechanical agents, and will, therefore, be taken to illustrate 
the general principles of pneumatics common to all elastic 
fluids. 

Atmospheric air is a thin, transparent fluid, which surrounds 
the earth, extending up from its surface to the distance of about 
forty-eight or fifty miles. By virtue of one of its constituent 
elements, it feeds the lungs, and gives vitality to the whole ani- 
mal creation, while it serves an agency scarcely less important 
in the sustenance and growth of the vegetable kingdom. The 
constitution and vital qualities of air are, however, proper sub- 
jects for chemical inquiry, its mechanical properties alone 
claiming attention in this connection. The obvious properties 
of air, such as its materiality, fluidity and elasticity, may be 
satisfactorily illustrated by a variety of mechanical contriv- 
ances. These, with the principles they explain, we shall 

Define Pneumatics. What are fluids ? How are fluids divided ? What 
elastic fluids are chiefly employed as mechanical agents ? What is atmospheric 
air ? What is said of the relations of air to the animal and vegetable creation ? 
What are the obvious properties of air ? 



88 INERTIA OF AIR. 

endeavor to present clearly, yet concisely, in the following 
experiments. 

MATERIALITY OF AIR. 

73. The proofs of its materiality are various, and among these 
is that furnished to the eye. 

Air is visible. — When seen through a great extent, as when 
we gaze at the firmament or a distant mountain, these present 
a faint blue, which is the color reflected by the great body of 
intervening air. Within a limited space, the light reflected is 
too feeble to give it color, and hence it is usually regarded as 
invisible. This is true of many of the more dense semi-trans- 
parent bodies. A glass basin of water, for instance, dipped 
from the ocean, appears colorless, while the same liquid seen 
through a great depth "off soundings " shows a peculiar dark 
green color. Thus, color, which is a characteristic of matter, 
belongs to air, in common with more dense bodies. 

When a body in motion meets with and sets in motion 
another body, the former loses a force equal to that which it 
imparts to the latter. This resistance of the body struck is 
termed its inertia (§ 4). Inertia or resistance is a property 
which can be predicated only of matter. Whatever, therefore, 
resists or destroys the force of bodies is itself material. 

74. The i7iertia of air shows itself in a variety of ways. If 
we stand upon the outside of a rail-car in the calmest day, the 
displacement of the air by our bodies, as w^e are borne through 
it, wdll offer a resistance equal to a stiff breeze blowing in the op- 
posite direction. So, when we attempt to carry a board, or any 
broad surface, exposed to the air, in the direction we are mov- 
ing, a powerful resistance is offered by this. 

Exjyeriment. — Place the Jloat-ivheel, Fig. 64, on the 
pin, so that the surface of the floats shall be in a line with 
the handle. Give motion to the wheel, and it soon ceases 



Is air material? The first proof stated of this ? What illustrations given ? 
Is inertia a property of material bodies ? Has air inertia ? What illustrations 
given ? State the experiment witli the float-wheel, Fig. 62. 



INERTIA OF AIR. 



89 




Fig. 65. 



Fig. 64. -j-Q revolve, owing to the resistance offered by 

the air. Now place the hub on the pin, so as 
J to bring the floats edgewise to the direction of 
motion, and the great diminution of the sur- 
face opposed to the air will cause the wheel 
to continue its revolutions much longer than 
before. 

One of the most satisfactory proofs of the 
materiality of air, from its resistance 
to moving bodies, is afforded by an 
arrangement seen in Pig. 65, and 
known as the vane and mill Two 
float-wheels are made to revolve in- 
dependent of each other, and precisely 
alike. Power is communicated to 
each, at the same instant, by means 
y^^ ^ ^\ of a rack, which plays into a small 

I a. \ pinion upon the shaft of each wheel. 

This rack is on a tube, which may be 
drawn up by means of a sliding-rod, 
and, when released, is suddenly forced 
down by a spiral spring within, giving 
a rapid motion to the wheels. 

Experiment a. — Screw the cen- 
tre-post to the hole of the pump- 
plate, and arrange the floats so that 
those of one wheel shall stand edge- 
wise, and those of the other facewise, 
to the direction of motion. Cover w^ith a bell-glass, having a 
brass cap and sliding-rod ; attach the loop upon the end of this 
rod to the hook at the top of the sliding-tube. Draw up, and 
suddenly depress this, which will give motion alike to both 
wheels ; but, owing to the presence of the resisting air, the one 




How may the materiality of air be shown with the vane and mill (Fig. 65) ? 
8* 



90 



GUINEA AND FEATHER GLASS. 



facewise will revolve only about one eighth as long as the one 
edgewise to the direction of motion. 

Now work the air-pump and remove the air from the bell- 
glass, and again give motion to the wheels, when both will be 
found to revolve very nearly alike, and much longer than either 
did before ; thus clearly proving that air is a resisting^ and of 
course a ^material body. 

It is the inertia of the air acting upon the wings of the 
feathered and insect tribes that enables them to fly and trans- 
port themselves through it from one place to another. This 
may be shown by placing some flies or other winged insects be- 
neath the receiver of the air-pump, and removing 
Fig. 66. ^\^Q a^ij,^ -vN^hen any efforts to rise by a use of the 
wings will be seen to be unavailing. 

75. Gravity acts alike on all bodies, light and 
heavy, and, were it not for the resistance ofiered 
by the atmosphere, a floe of cotton and a bullet 
let fall together from the same elevation would 
reach the ground in the same time. 

Experiment. — Let the tall Guinea and 
Feather Glass. Fig. 66, be well fitted to the 
pump-plate, and on each of the four small tables, 
beneath the brass plate which covers the top, 
place a dime and feather. Before exhausting the 
air from tlie glass receiver, turn the sliding-rod 
and button which supports these tables, until one 
of them drops, and lets fall its dime and feather ; 
while the former, by its weight, overcomes the 
resistance of the air, and falls rapidly, the lighter 
feather makes a tardy descent. Now produce a 
partial vacuum, and let drop a second table ; the 
removal of a portion of the resisting air will make the difier- 



How is it that birds are enabled to fly ? Illustrate in the case of flies and 
winged insects. State the experiment with the guinea and feather glass. What 
does this prove ? 



PHILOSOPHICAL WATER-HAMMER. 



91 



Fig. 67. 



ence less marked than before. Continue to exhaust, and the 
third will fall still more nearly together ; form a vacuum, and 
the fourth pair will show no perceptible difference in the time 
of their descent.* 

76. The fall of liquids in a vacuum^ 
and the agency of the atmosphere in break- 
ing the force of rain and hail as they 
descend from great elevations, may be 
illustrated by the PhilosopJdcal Water- 
Hammer^ Fig. 67. This is a strong glass 
tube, hermetrically sealed at one end, and 
provided w^ith a cup and stop-cock at the 
other. 

Experiment. — Remove the scop-cock, 
and fill the tube about half full of clear 
water ; replace this, and, by means of the 
coupler and hose, figures 51 and 63, connect with the air-pump, 
and with the stop-cock open, exhaust thoroughly, occasionally 
shaking the tube a trifie, to " churn out " the air that may 
still remain between the particles of the liquid. Turn the 
stop-cock and detach from the hose. If now the tube be 




* A more simple and economical mode of illustrating the same principle is 
by means of the long Guinea and Feather Tube, Fig. 60. Screw this to the 
centre hole of the pump-plate ; or, if too long and heavy, connect with this 
by means of the coupler and hose, figures 51 and 63. Before exhausting, sud- 
denly invert the tube, and mark the difierence in the fall of the light and heavy 
bodies. When a vacuum has been obtained, turn the stop-cock and remove 
from the pump, and again invert as before, "when the diflference in the fall of 
the light and heavy bodies will, as in the last experiment, be imperceptible. 
Now admit the air, and attach the condenser. Fig. 4G, by means of the coupler 
last used, and condense forty or more strokes, according to the size of the tube. 
Close the stop-cock and remove the condenser. The tube will now contain 
from two to four atmospheres, and the greater density of the air will be plainly 
visible in the greatly retarded fall of the light bodies ; thus most satisfactorily 
showing the materiality and resistance of air. 



What is the Philosophical Water-Hammer ? Give the experiment with this. 



92 



BUOYANCY OF AIR. 



held at an angle of forty-five degrees with the floor, and jerked 
so as to throw the liquid up two or three inches, meeting with 
no atmospheric resistance, it will fall with a hard, clhikinc. 
sound, and with the force of lead. ° 

From this experiment we learn one of the beneficial efiects 
of the atmosphere, in relieving the fall, and preventing the 
mjurious efiects which might otherwise result from the descent 
of rain, etc.; for, were there no atmosphere to impede the fall 
such bodies would strike the earth with the force of shot fired 
from a gun.^ 

77. The Buoyancy of ^ir. — Air being, like water, a ma- 
terial fiuid, tends like that to buoj up bodies in proportion to 
the amount displaced; hence, light and bulky bodies, as 
loose feathers, cotton and wool, weigh less in air than in a 
vacuum. 

Experiment. — Balance the thin glass 
globe by a weight, and, with the stop-cock 
closed, place the scales on the pump- 
plate, and cover with a receiver, as seen in 
Fig. 68. Exhaust the receiver, and the 
glass globe, which was before sustained in 
part by the fiuid air, will be now found to 
preponderate and weigh more than in the 
air. Now remove the globe, and balance 
in its place a bunch of feathers or cotton, 
and again exhaust the receiver, when these 

* A cheaper yet less perfect form of the Philosophical Water-Hammer, ex- 
hausted by heat and made permanently tight, is sold at the shops. Jerk the 
tube with caution and at an angle, lest the force of the liquid break through 
the bottom. No amount of exhaustion can free a vessel in which a liquid Is 
confined from a vapory atmosphere; and, as a vacuum is approached, 
ebullition goes on rapidly, although the liquid be near the freezing-point. 



Fig. 68. 




What may we learn from this experiment ? What is said in case there were 
no atmosphere to impede the fall of rain and hail ? What is said of the buoy- 
ancy of air ? Give the experiment for showing its buoyancy. 



IMPENETRABILITY OF AIR. ^3 

Tvill be found like the glass globe to indicate an increase of 
weight ; thus proving the truth of the common assertion that, 
bj the ordinary tests, " a pound of lead weighs more than a 
pound of feathers." 

Experiment. — This same principle may be illustrated 
without the use of the air-pump. Suspend, in the place of the 
glass globe in the last experiment, a small, light rubber bag pro- 
vided with a stop-cock ; balance this accurately, and then, with 
the condenser and coupler. Figs. 46 and 51, inflate the bag, close 
the stop-cock, and again suspend from the scale-beam, when it 
w^ill be found to have lost in weight, or be buoyed up by a force 
equal to the additional volume of air it now occupies. 

T8. Air is impenetrable. — This is perhaps, if possible, a more 
convincing proof of its materiality than any previously offered. 
It is a property of matter that no two bodies can occupy, the 
same space at the same time. To this proposition air conforms 
as strictly as lead or water. Attempt to force these into a ves- 
sel filled with air, and, although this may contract its limits and 
retire before the more dense intruders, yet still the space it 
really occupies can never be entered by a second body. 

Experiment. — Plunge an inverted tumbler or tall glass jar 
in a vessel of water ; the materiality of the air confined above 
the water will prevent this from rising and filling the jax or 
tumbler. 

The Diving-Bell acts upon this principle of the impenetra- 
bility of air. This is a large bell-shaped receiver, made strong 
and tight, and of sufiicient weight about the opening to cause 
it to sink to a great depth in water, and retain its proper posi- 
tion. In the upper part are provided seats for the workmen, 
while two tubes enter, one at the bottom, the other at the top ; 
the former connecting with a force-pump upon the wharf or 



Is impenetrability a property of matter ? Is air impenetrable ? What is 
said when we attempt to force other bodies into a space filled with air ? Give 
the experiment illustrating this. On what principle does the Diving-Bell 
act ? Describe the Diving-Bell. 



^^ WEIGHT OP AIE. 



deck Of the vessel, through which fresh air is supplied to the 
men in the bell, while the latter serves as an escape for the 
impure air. _ When lowered into the water, the air, a. in the 
experiment just given, precludes the entry of the water, and 
allows the workmen, although at a great depth below the sur- 
face, to operate without inconvenience. Again, if the air be 
removed from a vessel before it is plunged in water, as in the 
last experiment, the water will then enter and fill the entire 

Experiment a. — Suspend a small bell-glass 
beneath a larger, by means of a sliding-rod, and 
place them on the plate of the air-pump over 
ajar of water, as shown in Fig. 69. Lower the 
small bell upon the water; this refuses to enter the 
bell, owing to its being preoccupied by a second 
body — air. Raise now the bell, and exhaust the 
receiver, and again lower into the jar, when the 
water readily enters and fills it.* 

THE WEIGHT AND PRESSURE OF AIR. 

79. Since air is material, it possesses weight, like other more 
dense bodies; but, owing to its extreme thinness and trans- 
parency, this property in air becomes less obvious than in the 
grosser forms of matter. By the aid of skilful mechanical 
contrivances the weight of air may, however, be determined 
with as much precision as that of water or lead. The ancients 
although acquainted with the faot that air has weight, still 

Iva o H ' , ""''^^fon, owing to the impossibility of an entire re- 
moval of the a„. by any mechanical contrivance. This bubble of airwiU vary 
m size, acoordrng to the degree of the vacuum ^ 




State Experiment a. Has air wein-ht? Wl,v io „„f «• 
»ore„bvious. Can its weight be acc^rleiy 1^1;" ""^ "^^^""^ "' '^ 



WEIGHT OF AIR. 95 

kne^y little of its mechanical effects in producing the common 
phenomena of every-claj occurrence. Thus, the cause of the 
rise of water in pumps and other tubes was thought to be 
suction^ or the abhorrence which nature has of a vacuum ; 
hence, practising upon such vague and absurd maxims, they 
were constantly liable to those ridiculous blunders and expen- 
sive mistakes which -always accompany ignorance of physical 
laws. Galileo with his pupil Torricelli were the firsts to dis- 
cover the mechanical agencies of air in the rise of liquids, etc. 
Some engineers, employed at Florence in sinking pumps, had 
occasion to construct one to raise water from an unusual depth. 
Upon working it, they found the water would rise only to a 
height of about thirty feet. Galileo, the most celebrated 
philosopher of the day, was consulted as to the difficulty. 
Having his attention thus called to the point, his investigations 
with those of Torricelli soon disclosed the cause, and, further- 
more, produced the barometer as the measurer of the atmos- 
pheric weight. From these discoveries. Pneumatics as a 
science may be said to date. The following experiments, 
properly performed, will give some 
ideas of the weight of the air, and 
the surprising force with which it 
presses on bodies at the earth's sur- 
face. 

Experiment. — To loeigh a 
quantity of air. Take a pint or 
quart flask provided with a stop- 
cock ; connect this with the air- 
pump, and remove the air. Then 
close the stop-cock, and suspend the flask from a delicate scale- 




What is said of the knowledge of the ancients in regard to the mechanical 
effects of this property of air ? To what cause did they attribute the rise of 
liquids in pumps, etc. ? Who first discovered the real cause of these phe- 
nomena ? Anecdote in respect to the engineers and Galileo ? Give the ex- 
periment for determining the weight of air by exhaustion. 



96 



PRESSURE OF AIR. 



beam, as shown in Fig. 70. Now balance the flask by small 
weights, and when this is done admit the air into this. The 
side of the beam from which the flask is suspended will now 
be found to preponderate. Add small weights to the opposite 
scale-pan until the equilibrium is restored. These weights will 
indicate the weight of the admitted air, or of that removed hj 
the air-pump. 

A quart of air, at the ordinary density, weighs nearly 
seventeen grains. Attach now to the flask, by means of the 
coupler, Fig. 52, the condenser, and force in one or two addi- 
tional atmospheres ; a marked increase m the weight of the 
flask will now be perceptible.*" ' 

80. Since so small a quantity of air has appreciable weight, 
the pressure of the entire column reaching upward through the 
whole extent of atmosphere, and resting on a given surface, 
must be enormous. This is equal to about fifteen pounds on 
the square inch, or tv'o thousand one hundred and sixty 
pounds on the square foot. 

Experiment. — To prove atmospheric pressure^ place on 
the pump-plate the Hand- Glass, Fig. 71, 
and cover the smaller end with the palm. 
Exhaust, and as the air or support is taken 
from beneath, the weight of the atmospheric 
column resting dow^n from above will press 
the hand firmly to the glass with a force of 
from fifty to eighty pounds. 



Fig. 71. 




* Table showing the specific gravities of various gases, with the barometer at 
30 inches, and Fahrenheit's thermometer at 32^ : 



Air, 1.00 

Oxygen, 1.10 

Hydrogen, 0.06 

defiant Gas, 0.07 

Phos. Hydrogen, 1.21 

Nitrogen, 0.97 



Ammonia, 0.50 

Carbonic Acid, 1,50 

Prot. Oxide Nitrogen (Exhilarat- 
ing Gas), 1.52 

Chlorine, 2.47 



How by condensation ? What is the Aveight or pressure of the air on a 
square inch of surfiice ? A square foot ? 





PRESSURE OF AIR. 97 

Fig. 72. Experiment a. — Tie over the mouth of 

the Bladder- Cup^ Fig. 72, some highly elas- 
tic sheet-rubber, and screw, bj means of the 
connecting-screw, to the centre hole of the 
pump-plate. Exhaust, and the pressure of 
the external air will force in the rubber 
so as to form a perfect lining for the glass. 
Experiment b. — Connect the Cupping- 
Glass^ Fig. 73, with the air-pump, by means of the rubber 
hose, and place it on the arm or any part of the body where a 
slight puncture has been made in a vein. Exhaust, 
and the pressure of the air within, by its elastic 
force, will swell the skin, and cause the blood to 
spirt freely from the punctured vein. 

Remark. — It is by thus removing the external 

pressure that the leech is enabled to draw blood 

through the pores of the skin. All instances of 

suction are thus produced by the removal of the pressure of 

the external air by the air-pump, the mouth, etc. 

Experiment c. — Over the larger end of the Bladder- 
Glass^ Fig. 74, draw tightly and evenly ^ig. u. 
a piece of moistened hog's bladder, and 
fasten under the flange by means of a 
solution of gum-arabic or a string. 
When thoroughly dry, place on the 
pump-plate, and exhaust. The removal 
of the air or support from beneath will 
cause the weight of the external column 
to c?n/sh iji the bladder with a deafening report.^ 

* Should the bladder be so strong as to sustain the atmospheric pressure, a 
slight puncture with a pin will cause it to collapse as above. By inverting 
after the bladder is removed, this may be used, as in experiment, for a hand- 



Give Experiment a. Give Experiment b. State the experiment with the 
cuppmg-glass. Experiment with the bladder-glass. 

9 




08 



PRESSURE OF AIR. 




Experwient d. — Attach to the centre hole of the pump- 
plate the guard-screw. Fig. b%^ and on the plate place a thin 
glass bursting-square^ having a brass cap with a valve sealed 
upon the neck, like that seen in Fig. 75. Cover with 
a bell-glass, and exhaust ; the valve rises to let out 
the air. When a vacuum has been formed, admit 
the air again to the bell, and the valve closes 
tightly and preserves a vacuum in the square, 
which, having nothing within to counterbalance the 
external pressure, is crushed in by the atmos- 
pheric loeight.* 

81. Experiment a. — Place one 
hemisphere of the Magdeburgh Clips f 
on the pump-plate, as seen in Fig. 76, 
and exhaust; the pressure of the 
atmosphere will hold it firmly to the 
plate. 

Experiment b. — Remove the lower 
handle from the Magdeburgh Hemi- 
spheres^ Fig. 77, and screw the stop-cock to the centre hole 
of the pump-plate ; turn the upper hemisphere back and forth 
a trifle to insure a perfect contact of the two surfaces, 

* Care sliould be taken against scratching the plate when wiping from it 
the fragments of glass. As this experiment incurs some trouble and risk, 
and can be seen only by those immediately about the pump, it is hardly 
advisable to introduce it in ordinary lectures. The same cap and valve may 
be used with any number of squares. 

t These cups were invented and first used by Otto Guerick, the inventor of 
tlie air-pump, and named from the city where first used. An experiment was 
performed by the discoverer before the royal household with a pair of these 
cups, which were three feet in diameter. When exhausted of air, twenty strong 
horses attached, and drawing in opposite directions, ten upon each side, are 
stated to have been unable to separate the cups, so great was the force of the 
pressure of the air by which they were held together. 




Why is a thin square bottle crushed in when the air is removed from its in- 
terior ? 



MAGDEBURGH HEMISPHERES. 



99 



Fig. 78. 



and then form a vacuum. With the stop- 
cock closed, remove and replace the handle. 
These hemispheres, of five inches diam- 
eter, will now be pressed together by 
the force t)f the atmosphere^ so that two 

strong men may not be able 

to separate them. 

To prove that these hemi- 
spheres are now held together 

by the force of the air, let 

them be again screwed to 

the pump-plate, and covered 

by a bell-glass provided with 

a sliding-rod and hook, as 

seen in Fig. 78. Attach the 

hook of the rod to the handle, 
and exhaust the bell-glass ; the upper^ 
hemisphere may be now readily raised 
from the lower. Replace it carefully, and admit the air into 
the bell, when the hemispheres will be again held firmly together. 






Experiment b. — With the Magdeburgh Hemispheres ex- 
hausted as above, screw to the brass socket of the basement, 



What is the result in removing the air from within the Magdeburgh Hemi- 
Bpheres ? How can it be shown that it is the pressure of the air which holds 
these together so firmly ? Give the experiment for showing the amount of 
pressure on these. 



100 CONDENSING-CHAMBER. 

and arrange other ways, as shown by Fig. 79. In this way 
the pressure of a column of air with a base of the area of 
either hemisphere may be ascertained.* 

82. As has been ah-eady remarked, the pressure of the atmos- 
phere is equal, at the level of the ocean, to a weight of fifteen 
pounds to the square inch, and hence that sustained by the body 
of a common-sized man, comprising two thousand square inches, 
must be about fifteen tons. Such a weight resting on bodies 
might, at first thought, be expected to crush them, whereas we 
find those of the most delicate texture unaffected by it. This 
arises from the circumstance of the pressure on every side being 
equal, and, therefore, producing a mechanical equilibrium. 
Under ordinary circumstances, the pressure upon the external 
surface of the body is exactly counterbalanced by the elastic 
force of the air confined within its vessels and air-cells. When- 
ever this external pressure exceeds the elastic force within, as 
in sudden descents into deep mines, a heavy and oppressive sen- 
sation is the result. If, on the contrary, a speedy rarefaction 
of the external air be efiected, so that the elastic force of that 
within is more than sufficient to equal the pressure from without, 
as in ascending high mountains, or rising in balloons, a painful 
distension and even rupture of the veins and air-cells may occur. 

83. The effect on the animal system of an increase of atmos- 
pheric pressure may be satisfactorily shown by means of the 
Strong Glass Condensiiig- Chamber^ Fig. 80. This is a re- 

* Extreme caution is necessary against getting the least dirt on the flanges 
of these hemispheres, as such will prevent a perfect contact of the two sur- 
faces, and defeat the experiments. To remedy such a defect, carefully smooth 
down the roughness with a knife or fine file, and then grind together the 
flanges by the use of pumice-stone and oil, separating occasionally to prevent 
the formation of creases. 

What pressure does the body of a common-sized man sustain ? Why does 
not such a weight crush the body ? Cause of the oppressive feeling in case of 
descending into deep mines? Cause of the painful sensations upon ascending 
to great elevations ? Under ordinary circumstances how is the atmospheric 
pressure upon the external surface of the body counterbalanced ? 



CONDENSING-CHAMBER. 



101 



ceiver of about two quarts' capacity, made of thick and well 
annealed glass, and covered by a 
firmly cemented brass cap. Into 
this cap screws a large plug, 
which may be removed to admit ob- 
jects for experiment. In the centre 
of this plug is screwed a stop-cock 
for various attachments, when de- 
sired, for showing the expansive force 
of air, as in case of the air-gun, 
fire-engine, etc. A condensing- 
gauge. Fig. 48, for indicating the 
degree of pressure, is screwed upon 
the cap, and connects with the 
space within.* 

Experiment. — To show the ef- 
fect of an increase or diminution 
of atinospheric pressure on an animal. — Take two mice, 
or other small animals ; place one in the glass condensing- 
chamber^ and with the condenser force in three, four, or more 
atmospheres. The pressure will so compress the air-cells of 
the body as to crush in these and destroy life. Now place the 
other mouse under the receiver of the air-pump^ and exhaust, 
when its body will become bloated by the expansion of the 
veins, arteries, etc., and these loill be ruptured ; thus showing 
that, with the present physical organization, the atmosphere near 
the surface of the earth is precisely adapted to the wants of the 
animal system.f 

* Another and cheaper form of the condensing-gauge is shown standing 
"within the receiver. The long jet at the side is screwed to the lower end of 
the cock when water is used for illustrating jets, the fire-engine, etc. 

t Experiment. — Place in the chamber, the rubber-bag. Fig. 92. Inflate 
this, close its stop-cock, screw in the large plug, and attach the condenser. — 




State the experiment showing the effect of increased atmospheric pressure 
on animals. The experiment showing the effect of a diminution of the same 
on animals. 

9=* 



102 



POROSITY OF WOOD. 



Fig. 81. 




84. The operation of the lungs^ in inhaling and exhaling, 
depends on atmospheric pressure. The rise of the ribs and the 
fall of the diaphragm beneath, forming a partial vacuum in the 
chest, causes the air to flow in and expand these ; and, upon the 
rise of the diaphragm and return of the ribs, the same is forced 
from the lungs, thus sustaining, bj a wonderful mechanism, the 
process of respiration. 

Experiment. — To show the porosity of wood by atmos- 
pheric pressure, insert a straight-grained cylinder 
tightly in a brass plate, fitted to the top of the glass 
receiver, and arrange in a jar of water on the pump- 
plate, as in Fig. 81. Cover the end of the wood cyl- 
inder with the hand, until a vacuum is formed 
within ; then remove it, and the air will be forced 
rapidly through the pores of the wood, and rise 
through the water. 
Experiment a. — Place the Mercury- Tunnel^ Fig. 82, on 
the ground-top receiver, over a jar on the 
pump-plate, as seen in the cut. Pour mercury 
into the wood cup, and exhaust ; the atmos- 
pheric j^ressure will force this down through 
the pores of the wood in a fine shower, and, 
should the air be dry, the friction of the 
mercury will excite sufficient electricity to 
render it luminous in the dark.* 

85. Experiment. — The atmospheric pres- 
svre upon liquids^ as in the case of their 

Force in one atmosphere, and the bag will be compressed one half. Three or 
four atmospheres will so compress the air within as to give the bag a shrivelled 
appearance. Allow the condensed air to escape, and the bag becomes inflated. 

Experiment. — Place in the condensing-chamber two small bursting- 
squares, one with an open, and the other with a sealed muzzle. Condense, and 
the sealed squai'e will be crushed in, while the other receives no injury. 

* Guard against letting the mercury fall on the pump-plate. Keep this 
mercury-tunnel entirely free from water and other liquids. 

On what principle do the lungs operate? How may the porosity of wood be 
Bhown by Fig. 79 ? How by the Mercury-Tunnel, Fig. 82 ? 




"^^^ 



BOLT-HEAD. 



103 



Fig. 83. 




rise in exhausted tubes and receivers, may be illustrated bj an 

arrangement shown in Fig. 83. A tall glass holt-head^ of about 

one pint or quart capacity, and thirty inches length. 

is screwed to the cup of a bell-glass. From the 

lower end of the stop-cock a small tube extends to 

the bottom of a jar containing some colored liquid, 

just sufficient to fill the bolt-head. Arrange on the 

pump-plate, as shown in the figure, and exhaust 

thoroughly. The air will be drawn down from the 

bolt-head through the small tube, and prevented 

from returning again by the liquid. Now open 

the vent-hole, and the atmospheric pressure 

will drive up the liquid to occupy the space from 

which the air has been removed, leaving only a 

bubble in the glass bulb, and illustrating the 

^ materiality as well as the pressure of the air. 

\^f^ To remove the bolt-head, =^ exhaust again, and 

bring the liquid below the stop-cock ; then 

turn this and unscrew from the bell-glass. f 

Experiment a. — Take a tall and straight jar, and fill to the 

* When tlie bulb is filled with liquid, the pump should be worked steady, 
lest the glass neck be broken off by the motion. This experiment succeeds 
best where regard is paid to the hydrostatic pressure on the air escaping from 
the tube. A broad jar, and liquid just sufficient to allow the tube to enter 
when the bolt-head is filled, is preferable. 

Fig. 84. f An interesting modification of this experiment is 

shown by the arrangement in Fig. 84. Within the bell- 
glass, to the lower end of the stop-cock, is screwed a 
revolving-jet, so as to revolve in an empty jar be- 
neath. A tube, leading from the upper end of the stop- 
cock, enters a jar of water. With the stop-cock closed, 
exhaust the bell-glass, then open ; the atmospheric pres- 
sure will drive over the liquid through the tube and 
jet, causing the latter to revolve rapidly for some time, 
thus illustrating the cause of the rise of liquids, as well 
as the fact that Barker^ s Mill (see Hydrostatics) re- 
volves equally as well in a vacuum, and independent of atmospheric reaction. 




Describe the experiment with the bolt-head. What does this illustrate ? 



104 



SIPIIOX-BAROMETER. 



Fig. 85. 



brim with water ; then place over the open end a piece of thick 
writing-paper, pressing this carefully against the glass rim, 
and then invert, as seen by Fig. 85. The at- 
mospheric 2)ressiire will hold the paper against 
the water, so that the jar may be carried about 
the room without the liquid falling. 

86. Experiment. — To show the height to 
which a column of mercury may be " pumped " 
or forced up an exhausted tube by the weight of 
the atmosphere : place the lower extremity of the 
siphon. Fig. 86, in a cup of mercury, ri?. 86. 
and. with the stop-cock open, connect /^^ 
with the air-pump by means of the 
rubber hose, and exhaust thoroughly. 
Near the level of the ocean, where the 
mercury sustains the pressure of the 
—^entire atmospheric column, it will 



rise in the tube, and stand at an ele- 
vation of about thirty inches.^- 

Since, therefore, the weight of a column of atmos- 
pheric air is equal to a column of mercury thirty 
inches high resting on the same base, the iveight 
of the entire atmosphere is equal to a sea of mer- 
cury covering the whole surface of the globe to the 
depth of thirty inches. This, from a careful es- 
timate of the superficial area of the earth, is found 
to be nearly ^z;e thousand hiUions of tons. 

By the last experiment, mercury, which is about thirteen and 



* Guard against raising the end of the tube from the mercury -while the 
cock is open, lest the entry of the air force the fluid over into the air-pump. 
The same is well shown by the barometer attached to the large pumps. 



How may atmospheric pressure be illustrated by the inverted jar of water, 
Fig. 83 ? How high will the weight of the atmosphere raise and sustain a column 
of mercury ? To what is the entire weight of the atmosphere equal ? 



ABSURDITY OF SUCTION. 



105 



one half times heavier than water, was seen to rise by atmos- 
pheric pressure to the height of thirty inches. By the same 
pressure, water will, of course, be raised thirteen and one half times 
as high, or to an elevation of nearly thirty-four feet. 
In practice, however, water will rise in an exhausted 
tube only about twenty-eight or thirty feet, owing to 
the rapid formation of a vapory atmosphere above 
the liquid when subjected to a vacuum. 

87. The absurdity of suction^ as a cause of the 
rise of fluids in exhausted tubes, may be clearly shown 
by Fig. 87. Let a glass tube be screwed to the lower 
end of the stop-cock of a Torricellian receiver, and 
extend down into a cup of mercury, as seen in the 
figure. To the upper end of the stop-cock screw 
an exhausting-syringe, and place the whole on the 
plate of the air-pump, and perfect a vacuum in the 
receiver. Now with the stop-cock open work the ex- 
hausting-syringe ; the mercury cannot be sucked 
up in the barometer-tube ; but admit the air into 
the receiver, and then work the syringe, and the 
mercury rises by atmospheric pressure on the fluid in the cup, 
which could not happen when this pressure was removed by an 
exhaustion of the receiver.* 

* Ignorance of pneumatic laws has occasionally produced ridiculous and 
expensive mistakes, as in the construction of tight wells, the leading of siphons 
over elevations more than thirty feet, etc. A celebrated Scotch philosopher 
relates that a wealthy friend had occasion to construct a pump after the most 
thorough manner. When completed, to the great surprise of the owner it 
failed to operate, and, upon inquiry, he informed his scientific friend, to whom 
he applied for an explanation of the cause of the failure, that he had taken 
great pains with the whole, having even the walls and top of the well tightly 
covered with cement. The cause of the difficulty was at once suspected, and an 
opening through the top for the admission of the air suggested, which proved 
an eflectual remedy, causing at once a successful operation of the pump. 



If mercui'y rise by atmospheric pressure to the height of thirty inches, how 
high may water be raised by the same pressure ? Give the experiment for 
showing the absurdity of suction. 



106 



UPWARD PRESSURE OF AIR. 



Fig. 88. 



88. It is by atmospheric pressure that flies and other insects 
are enabled to walk up smooth glass, and inverted on the ceiling, 
and that limpets and shell-jish are held so firmly to the sides of 
vessels and rocks ; the feet of such being formed for expelling the 
air, and producing a partial vacuum 
beneath, whereby they are held by the 
force of the air to the objects upon 
which they fix. This may be illus- 
trated by 

Experiment. — To the centre of a 
circular piece of smooth leather fasten 
a string. Wet the leather, and press it 
against the smooth surface of a pebble 
or piece of metal. Upon taking hold 
of the string a partial vacuum will 
be produced by the raising of the 
leather in its centre, and the atmos- 
pheric pressure will hold it to the 
pebble, etc., so that the body may 
be suspended in the air. 

Air is governed by the same laws 
as the more dense fluids, pressing in 
all directions, upwards as well as 
downwards. 

Experiment a. — Draw the piston 
to the bottom of the glass cylinder of 
the upward pressure apparatus^ 
Fig. 88, and attach a fifty-six, or 
greater weight; not, however, so as 
to hang at all on the piston when at the bottom. Screw the 
hose to the pump-plate, then to the small plate on the glass 
cylinder, as shown in the figure. Exhaust, and the pressure of 
the external air will drive up the piston into the vacuum 




In what directions does the air exert its pressure ? How may this upward 
pressure of the air be shown by Fig. 88 ? 



THE BAROMETER. 



107 



above, taking up also the attached weight. Now pull down upon 
the leather strap and let go the hand ; the piston and weight 
will rise and tilt on the air several times, forming a beautiful 
atmospheric spring * 

89. The Barometer. \ — The weight of the atmosphere, and 
its power of balancing a column of a more dense fluid, 
pressing down with an equal weight, has been already illus- 
trated (Fig. 86). It is on this principle that the barometer 
operates. Take a strong glass tube, hermetically sealed at one 
end and open at the other ; fill this with mercury, and invert it 
Fig. 89. in a cup of the same fluid, and the mercury 

will fall dow^n so as to stand at about thirty 
inches, leaving a vacuum in the upper portion 
•J jjX of the tube.f At this height it is supported by 

* Observe the order in the aboye description. Guard 
against letting the piston come up violently against the 
brass plate, so as to raise it and admit the air suddenly, 
as in such case the weight "will fall heavily upon the floor. 
The piston should never be taken from the cylinder by 
the inexperienced, as such will be apt to injure the nice 
edge of the leather in replacing. If the hose is stiff, guard 
against letting the brass plate tip off the cylinder upon 
the surrounding glasses. 

t Baoog, weight, and /listqov, a measure, 
t In filling these tubes heat is usually applied to expel 
the air which adheres to the sides of the glass, or is lodged 
between the particles of mercury. A more successful method 
of filling barometer-tubes, however, is that devised by Mr. 
Chamberlain, of Boston, and shown by Fig. 89. The tube 
is placed, with its open end up, beneath a tall receiver, a, 
provided with a funnel and stop-cock, b. Into this tube 
enters the stem, s, of the stop-cock. The lower and sealed 
end of this tube rests on a cork, c, so that the tube may 
be tilted without harm during the operation. Pour some 
pure mercury into the tunnel, after the apparatus is arranged on the pump- 
plate, and with a good air-pump exhaust thoroughly ; then open the cock. 




On what principle does the barometer act ? Describe the manner of making 
a simple form of the barometer. 



108 THE BAROMETER. 

the atmospheric pressure. If this tube be allowed to remain 
in an upright position, and carefully watched, the mercurial 
column will be seen to vary from time to time, corresponding 
"with the changes in the density and pressure of the air. Let 
now a scale, graduated and marked to indicate these changes, 
be afl&xed to the tube, and we have the common barometer. 

Experiment. — The principle on which the mer- 

Fig. 90. ^ ^ ^ 

curial column is supported, and rises and falls in 
the barometer, may be illustrated by Fig. 90. 
Place a common barometer-tube on the plate of the 
air-pump, and cover with a tall Torricellian 
receiver. Before exhausting, the pressure of the 
air on the surface of the mercury in the cistern 
will support this in the tube at about thirty 
inches.* Work the pump, and remove a portion 
of the air from the receiver, and the mercury falls 
in the tube to correspond with the rarefaction of 
the air, or diminution of its density and pressure. 
Continue to exhaust, and the mercury will rapidly 
descend, and soon very nearly reach a level with 
that in the cistern. Admit the air, and it again 
rises and stands as before. 

90.,. The barometer may, therefore, be regarded as a measure 

and let the mercury drip slowly into the tube until it is filled ; remove the 
receiver, and invert and place the end of the tube in a mercury-cistern. The 
barometer tube is thus freed from air more effectually than by any other 
knoAvn method. 

♦Mercury is about 11,152 times heavier than air; and, since different 
fluids which balance each other have their heights inversely as their gravi- 
ties, it follows that the height of a column of air, of uniform density, which 
will sustain the mercury in the barometer at 30 inches, will be 30 X 11,152 
= 334,560 inches, or 27,880 feet= 5.28 miles. This, then, may be regarded 
as the average height of the atmosphere, if it be supposed of uniform 
density. 

How may it be proved that it is the pressure of the air which supports a 
column of mercury in the barometer ? Of what is the barometer a measure ? 



THE BAROMETER. 109 

of the weight and density of the atmosphere ; and, as these varia- 
tions of density correspond somewhat to the changes of 
weather, this instrument is usually regarded as a weather- 
glass^ and marked on the scale accordingly, rain^ f(^ii^^ and 
dry. Such marks are, however, arbitrary, and far from afford- 
ing an infallible guide ; for, while one barometer on the sea- 
shore, which sustains the weight of the entire atmospheric column, 
indicates fair weather, another, on a high eminence near by, 
above the lower stratum of air, and, of course, sustaining less 
of its weight, may, at the same time, indicate rain, or even a 
violent tempest. In general, however, the rise of the barometer 
denotes fair, and its descent stormy weather. A sudden and 
remarkable fall of the mercury usually precedes and attends 
violent winds : hence this instrument is of the highest service 
to mariners when sailing in tropical latitudes, where they are 
liable to meet with whirlwinds, typhoons, and other fearful 
winds ; the barometer often giving sufficient warning to enable 
them to avert these dangers. 

The barometer is also of great value in determining heights. 
The density of the air diminishes regularly as we ascend from 
the level of the ocean ; thus, at three miles it is only about one 
half ; at seven miles, one fourth, and so on. Now, as the height 
of the mercury corresponds to the density of the air, and this to 
the elevation, the height of any place may be readily known by 
noting the point at wdiicli the mercury stands at that place. 
Thus, when ascending a hill or mountain, or rising in a bal- 
loon, the elevation may be known at any time by consulting the 
barometer. If this, for instance, stand at 24.79 inches, the 



Why do the rise and fall of the mercury in this usually indicate changes of 
weather ? How is the scale of the barometer usually marked ? What is said 
of these marks ? Why are they not unerring guides in determining the states 
of weather? In general, what does the rise of the mercury indicate ? What 
its fall ? What does a sudden fall of the mercury indicate ? Where is the 
barometer of special service ? How are we enabled to determine heights by 
means of the barometer ? 

10 



110 



BOILING OP LIQUIDS. 



elevation is nearly 5,000 feet ; if at fifteen inches, 10,000 
feet ; and so diminishing with the elevation. 

91. The Boiling of Liquids. — The temperature at which 
liquids boil varies with the pressure on these. Thus, under the 
ordinary atmospheric pressure of fifteen pounds to the square 
inch, water boils at 212° Fahrenheit. If, now, the same be 
placed in a receiver, and a partial vacuum effected 
by means of the air-pump, the degree of heat re- 
quired for making it boil w^ill become less ; and 
in the vacuum produced by a superior modern 
air-pump the liquid will boil even at the freez- 
ing-point of w^ater. On the contrary, water may 
be heated, under a pressure, so hot as to melt 
lead^ and yet not boil. 

Exjieriment. — Take a strong glass flask pro- 
vided with a stop-cock, as seen in Fig. 91 ; place 
in this a thermometer prepared for the purpose. 
Pour in some water, and, with the stop-cock open, 
apply heat. When the thermometer indicates 
212°, the elasticity of the vapor will overcome 
the atmospheric pressure, and ebullition will begin. 
Close the stop-cock so as to prevent the escape of 
the vapor ; a greater pressure will at once stop 
the boiling, and, if the heat be continued, the 
thermometer will rise several degrees before this 
will commence again. Now remove the lamp, and 
allow the liquid to cool considerably below 212°, 
and, with the stop-cock closed, plunge the flask in cold water., 
when a rapid boiling will be renewed, owing to the formation of 
a partial vacuum by the condensation of the vapor above the 



How docs the temperature at which liquids boil vary ? The boiling-point 
under the ordinary pressure of the atmosphere ? What is said of the boiling- 
point in an exhausted receiver ? Describe the process of boiling in the glass 
flask, Fig. 91. Why does the liquid again boil when the flask is plunged in 
cold water ? 



ATMOSPHERIC TELEGRAPH. Ill 

water in the flask. This boiling by cooling may be repeated 
several times, and constitutes the cidinary paradox.* From 
the same cause liquids boil at a much lower heat on mountains 
than near the level of the ocean. Thus, on the summit of Mont 
Blanc, the boiling-point of water is onlj 180°, a temperature 
too low for cooking many common vegetables. 

Upon this same principle, Mr. Howard, of England, a few 
years since, devised a plan for boiling down syrups at a low 
temperature, by placing these in large vacuiim-pans^ from 
which the air and vapor may be constantly removed by air- 
pumps. By this arrangement much less heat is required, and 
the quality of the sugars thereby greatly improved. 

Experiment a. — Water at different te^nperattires boils 
at different degrees of rarefaction. — Fill the tall jar, Fig. 
85, about half full of cold water, then pour in carefully, 
through a long tunnel, a pint of hot water. Cover with the 
bell-glass, and exhaust, and the warm water will boil violently, 
w^hile no visible effect will be produced on the cold water 
below. 

92. The Atmospheric Telegraphy an invention recently 
patented by Mr. I. S. Richardson, is designed for transmitting 
mails and other matter, at great speed, through exhausted 
tubes, by the force of atmospheric pressure. A piston or 
plunger, packed with soft leather, fits the tube; to this is 
attached a long cylindrical mail-bag. When ready for trans- 
mission, the plunger with its attachments is placed in the 
end of the tube behind an air-tight " cut-off." The tube is 
then exhausted by large air-pumps, to be worked by steam 

* A spring gauge, for showing the degree of the pressure of the yapor, is 
screwed to the cock, as seen in the figure. This may be removed. 



What is said of the boiling-point of liquids on mountains ? The boiling- 
point on Mont Blanc ? The use and advantages of vacuum-pans ? Give Ex- 
periment a. What is the design of the Atmospheric Telegraph? How is it 
operated ? 



112 FLUIDITY OF AIR. 

power ; and when a tolerable vacuum is effected, the '' cut-off" 
is raised, and the plunger set free on the side of the vacuum. 
Atmospheric pressure then forces this into and through the 
exhausted tube " with a speed equal to about six hundred and 
thirtj-five miles per hour." Although highly operative on a 
small plan, as shown by models, yet the practicability of the 
Atmospheric Telegraph on a large scale remains to be tested. 

FLUIDITY OF AIR. 

93. Air is a fluid, and follows the general laws which govern 
the flow and equilibrium of water and the more dense fluids. 
Thus, when a space is rendered partially void — as the receiver 
of an air-pump, for instance — by exhaustion, the fluid air 
around, when permitted, at once flows in to fill the empty 
space. So, on a more extended scale, the rarefaction of the air 
over a certain region by the sun's heat, causes the surrounding 
denser portions to flow in, forming the gentle zephyr as well as 
the terrific tornado. 

Again ; air, like the more dense fluids, raises and floats 
bodies specifically lighter than itself Thus, as water raises 
and floats wood, cork, etc., so air raises and buoys up a balloon 
inflated with hydrogen, which is specifically lighter than itself 
In the same way clouds and smoke are lifted and sustained 
by the air. 

ELASTICITY AND EXPAXSION OF AIR. 

94. Of elastic fluids air may be taken as a type or example, 
being permanently elastic. If a portion of this be subjected to 
the most intense pressure for years, it loses none of its elasticity, 
but at once resumes its original volume Avhen this pressure is 
removed. 



The speed at which it is proposed to transmit packages by this ? Is air a 
fluid ? Give any illustrations of this. Of what class of fluids may air be 
taken as the type ? What is said of its elasticity ? 



EXPANSION OF AIR. 113 

As we have already remarked (90), tlie density of the 
atmosphere diminishes as we ascend from the earth, where the 
pressure of the incumbent mass becomes less and less. This is 
agreeable with the law of Marlotte^ that " The volumes of 
gases are in the inverse ratio of the pressures which theij 
support. ^^ 

Experiment. — Take the siphon-harometer^ Fig. 86, re- 
move the hose, and, with the stop-cock open, pour in mercury 
through the long arm until it shall stand just above the curve 
in either arm. Close the stop-cock ; the air in the short arm 
now sustains the ordinary pressure of a single atmosphere. 
Pour mercury into the long arm until it shall stand at the 
same height above that in the short arm as that in the barom- 
eter. The air confined in the short arm now sustains a weight 
equal to two atmospheres, and has diminished in bulk one half. 
The weight of an additional atmosphere would cause it to oc- 
cupy one third its original volume, and so with the pressure of 
each additional atmosphere will be a proportional diminution 
of volume, agreeably with the above law. 

95. The expansion of air has no assignable limits^ but, as 
the pressure is removed, the want of cohesion 
between its particles causes them to separate, 
and the fluid to occupy an indefinite volume. 
Experiment a. — After pressing the air as 
far as possible from the sheet-rubber bag^ 
screw into the nozzle a hook-plug, to make it 
air-tight, and suspend it under a bell-glass from 
the stop-cock, as seen in Fig. 92, or, which 
is better, from a sliding-rod. Place the whole 
on the pump-plate, and exhaust ; the rare- 
faction in the receiver will cause the trifle of air remaining in 
the bag to expand and swell it to plumpness ; and, if the 

What is tlie law of Mariotte ? How may this be illustrated by the siphon- 
barometer, Fig. 86 ? What is said of the expansion of the air ? Show the ex- 
pansive force of air by the arrangement, Fig. 92. 

10* 




114 



PNEUMATIC BALLOON. 



J 



bag be not sufficiently yielding, the expansive force may be so 
great as to burst it. Thus, the expansive force of the air con- 
fined -within shrivelled apples, raisins, etc., will cause them, 
when placed in an exhausted receiver, to become plump and 
full, and appear as when fresh. 

Experiment b. — Screw into the nozzle of a small rubber 
bag a screw-plug, after the air has been 
j-£ forced from it by the hand; attach a weight, 

f^i place in a tall jar of water, and cover with a bell- 
glass on the pump-plate, as seen in Fig. 93. 
Atmospheric pressure acts through the water on 
the air confined in the bag. Remove this pressure 
by exhausting, and the bag at once 
becomes inflated by the expansion of '°' 

its confined air, and rises. Admit 
the air into the bell, and the bag 
again contracts, becomes specifically 
heavier than the water, and descends, 
thus illustrating the Hydrostatic 
Balloon. 

Experiment c. — The same prin- 
ciple of aerial pressure and elasticity 
is better illustrated by the Pneumatic Balloon^ 
Fig. 94. This balloon is placed in a tall jar, nearly 
filled with water or alcohol, with a covering of 
sheet-rubber tightly drawn over its top, and is 
partly filled with liquid, leaving a portion of air "* ' *" 

confined in the upper part, sufiicient to render it 
a trifle lighter than the same bulk of the liquid. When float- 
ing upon the top, a pressure on the sheet-rubber will be com- 
municated through the air to the liquid, which will be affected 
throughout ; and, by its upward pressure, enter the small hole in 
the bottom of the balloon, compressing the air confined in it, 



Explain the principle on •wliich the Hydrostatic Balloon rises and sinks, as 
seen in Fig. 9.3. Explain the Pneumatic Balloon, Fig. 91. 



ELASTICITY OF AIR. 115 

and causing the balloon to sink, from an increase of its specific 
gravity. Remove the pressure, and the balloon rises.* 

Remarks. — This instrument illustrates several important 
principles in pneumatics and hydrostatics. By the materiality 
of the air, which intervenes, pressure is communicated to the 
water ; the co7nj)ressibilitij of the air in the balloon causes it to 
sink : its elasticity expels the water again from this, causing it to 
rise, while the lightness of the air buoys up the balloon. The 
pressure of liquids is in all directio7is, for the water rises in 
the balloon, while the pressure on it is downward, and their 
pressure is as the depth, for the air is diminished in bulk as 
the balloon descends. 

On the principles illustrated by the last experiment, j^^/ie^ rise 
and sink in ivater. These are provided with an air-bladder, 
lying along the vertebral column, by a voluntary contraction 
and expansion of which, they are enabled to change their spe- 
cific gravity, so as to descend or rise at pleasure. 

Experimeyit c. — Fill the condensing-chamber, Fig. 80, 

* The pressure required to sink this -will diminish as the depth of its descent 
increases, and when nicely balanced at the top, and forced to the bottom, it 
will remain there, from the pressure of the water above on the air confined 
within the balloon. To raise it again, place the jar under a tall bell-glass. 
Fig. 91, and exhaust a trifle ; or take a long slim tube, with a curved end, 
pass it down into the jar, and bring to the small hole in the bottom of the bal- 
loon ; then apply the mouth to the tube, and blow in air until the balloon 



^ 



a 



The expansive force of air may be shown by screwing into the 
hole of the pump-plate the guard-plug, Fig. 54, and placing 
on the plate, beneath a wire-guard, a bursting-square. Fig. 95, 
the nozzle of which has been closed with sealing-wax. Cover 
with a bell-glass, and exhaust. The removal of the external 
pressure will cause the air confined within to exert its ex- 
pansive force without a counterbalance, and the glass will 
burst, flying into a thousand pieces. Observe the caution 
in § 80. 



What important principles are illustrated by this experiment ? Cause of the 
rise and fall of fishes in water ? Illustration with the condensing-chamber ? 



116 



BACCHUS ILLUSTRATION. 



96. Experiment. 



nearly full of water, and place in it a fish; remove the 
condensing-gauge, and in its place screw a plug ; connect with 
the air-piimip by means of a hose and coupler, and exhaust. 
The removal of the pressure from the water, and through this 
from the air-bladder, Avill cause this to expand, and bring the 
fish to the top of the water ; and, if the rarefaction be carried 
sufiiciently far, the bladder will be ruptured. Again, attach the 
condenser., and work it a few strokes, and the compression of 
the air in the bladder will bring the fish to the bottom, in spite 
of its efforts to rise. 

— Bacchus m Vacuo, Fig. 96, is an amus- 
ing illustration of the expansion of air. 
This is an image mounted on a small 
barrel, which has a partition in the 
middle dividing it in two parts, one of 
which is open to atmospheric pressure, 
while the other is closed, and contains 
some colored liquid, with a small portion 
of confined air above it. From the end 
of the barrel in which the liquid is, a 
glass tube passes into the mouth of the 
image, and down through it to the open 
end of the barrel. Upon covering with 
a bell-glass, and exhausting, the expayi- 
sion of the air in the closed end of the barrel forces the 
liquid up the glass into the mouth of the' image, while, at the 
same time, a tight rubber bag concealed under the dress, ex- 
pands and swells the abdomen, thus adding to the illusion. 

Experiment a. — The elastic force of air acting on liquids 
is shown by the Fountain in Vacuo, Fig. 97. Fill the lower 
globe nearly full of colored water, and let the centre and open- 




In the experiment of Bacchus in Vacuo, what causes the liquid to rise in the 
tube leading from the barrel to the mouth ? Cause of the expansion of the bag 
attached to the image ? What causes the liquid to flow from the lower to the 
upper globe, in Fig. 97 ? 



PNEUMATIC FOUNTAIN. 



IIT 



Fig. 97. 



mouthed globe with its jets be screwed into this, while the upper, 
with its neck down, covers the jet in the 
centre globe. Place on the pump-plate, 
and cover with a bell-glass ; exhaust, and 
remove the pressure of the air from the 
open jet. The expansive force of the 
air in the lower globe will drive the water 
into the upper, from which it falls into the 
centre globe. Now admit the air, and the 
water is all driven up, and remains in the 
upper globe. Exhaust, and it again falls 
into the centre one.^. 

97. The expansive force of the air is 
directly as the pressure upon it. — Remove 
the exterior jet of the Artificial Fountain- 
glass, Fig 99. Screw to the centre hole of the pump-plate, 
and exhaust ; replace the jet, immerse the end in water, 
and open the stop-cock. The pressure of the external air will 
force in the water through the long inner jet, forming a beau- 
tiful vacuum fountain. When about two thirds filled, attach the 





* A revolving-jet and fountain in vacuo, beautifully 
illustrates the same property of air acting on liquids. 
Attach a revolving-jet to the lower globe in the last exper- 
iment, and arrange on pump-plate, as seen in Fig. 96. 
Exhaust, and the expansive force of the small body of air 
confined above the water will act on this, and force it up 
through the jet, causing this to revolve rapidly, indepen- 
dent of atmospheric reaction. 

Keep these revolving-jets well oiled, and regulate their 
freedom by the binding-screw at the end. After such ex- 
periments, requiring the use of water, brass-work should 
never be set away in a damp and neglected state. More 
apparatus is injured by carelessness and neglect, than 
by use. 



Why, upon admitting the air again into the receiver, does the liquid flow 
from the middle into the upper globe ? What is the expansive force of air as ? 
Give the experiments with the Artificial Fountain-glass. 



118 



PNEUMATIC FOUNTAIN. 



Fig. 100. 



condenser, by means of the coupler. Fig. 51, and work it twelve, 
fifteen, or more strokes, ac- 
'^' ^ ■ cording to the space of 
'^ air. Turn the stop-cock, 

remove the condenser, and 
attach again the short jet, 
or the hose, Fig. 101. Open 
the stop-cock, Sind. t/ie elas- 
tic force of the condensed 
air will drive out the 
water in a stream to the 
distance of thirty feet or 
more, according to the pres- 
sure. Close the stop-cock 
and attach the revolving- 
jet ; this will revolve rap- 
idly, emitting a broad circle 
of spray. 

Experiment. — Fill the 
strong Copper Condensing- 
chamber^ Fig. 100, about 

three fourths full of water, insert the stop-cock, 5, 
with the interior jet, 4 ; and, with the side-cock, 2, closed, 
attach the condenser to 5, and work it twenty or more strokes. 
Close this stop-cock, remove the condenser, and screw the short 
jet into the upper end of the stop-cock ; screw on the water-pan, 
6, with its tube, then upon this, the tunnel, 7, and drop into it a 
small light wooden ball. By means of a set of wires, 8, this ball 
will rest directly over the jet. Now open the stop-cock grad- 
ually, and the water will form a beautiful Je^ d'eau^ taking up 
with it the ball, which will be supported on the crown of the 
jet, at an elevation of several feet.* 

* The jet should stand perpendicular, so that the stream will flow directly 
through the centre of the pan. The ball, upon falling, when out of the centre, 
will be adjusted by the wires, 8, and carried up again. With the chamber 
fi-ce from water the ball, will be sustained hy a jet of air, though for a less time. 




AIR-GUN. 119 

98. Experiment. — With the condensing-chamber charged, as 
in the last experiment, attach the i/o^e, Fig. 101, 
Fig. 101. ^^ ^^^ side-cock. Water may thus be thrown to ^'°" ^^'^' 
a surprising distance, affording a good illustra- 
tion of the theory of the Fire-Engine. 

Exj^eriment a. — The Air- Gun is an instru- 
ment for throwing bullets, etc., by the elastic 
force of condensed air. This may be illustrated 
by screwing to the side stop-cock * of the con- 
densing-chamber, when highly charged, the 
Aii^-Barrel^ Fig. 102, and, placing in this a 
nicely-fitting lead ball, or some peas. Turn the 
key of the stop-cock half rounds as quick as 
possible. Only a small quantity of air will 
escape, but with such force as to drive out and lodge iSF 
the ball in a board, as when fired from a powder- 
gun. This may be repeated several times before the elastic force 
of the ail' will be perceptibly spent. If the condensing-gauge 
be screwed to the stop-cock, 5, and this 

Fig. 103. . . ^ ' ' 

then opened, it will show the pressure and 

qua-ntity of air escaping at each successive 

discharge. 

Experiment h. — With the chamber 

highly charged, screw to the side stop-cock 

the Revolving-Jet^ Fig. 103. The elastic 
force of the escaping air will drive this with surprising 
velocity, producing at the same time a peculiar singing 
sound. 

Experim^ent c. — Attach to the condensing-chamber the 
plate Pneumatic Paradox., Fig. 104. Upon this plate place 

* This stop-cock should have through it a larger hole than usual. All the 
cocks and joints about a condensing-chamber should be absolutely tight. 

What is the Air-Gun ? Explain the manner of illustrating this. Give the 
experiment with the Revolving-Jet. 




120 



PNEUMATIC PARADOX. 



Fig. lOi. 



a thin circular mica or paper disc, somewhat less in diameter 
than the plate, with a pin passing 
through its centre down into the tube, 
to keep it in place. Open the stop-cock, 
and, however great be the force of the 
escaping air, it will not be sufficient to 
blow this light disc from the plate. In 
an exhausted receiver^ however, this is 
readily blown off. 

Experiment d. — Again, attach the 
Pij^e Paradox^ Fig. 105, and place in 
the bowl a small, light wood ball. Pro- 
ceed as before, and the ball will remain in the 

bowl, in spite of the great force of the escaping air. The 
^. , „ result will be the same if the mouth of 

Fig. 106. 

Q^ the bowl be turned down while the air is 

/^ ^\ discharging. 

Experiment e. — The expansion of the 
bubbles of a gas escaping from a liquid 
may be shown by pouring one or two gills 
of ferm^enting beer or porter into a tall jar. 
Fig. 106, and covering with the bell-glass 
on the plate of the air-pump. When so 
arranged, exhaust, and the bubbles of escap- 
ing gas will expand so as to fill the entire 
-J jar with a delicious foam.=^ 



^ 



^ 



MECHANICAL AGENCY OF AIR IN RAISING WATER. 

99. The weight of the atmosphere, and its power in raising 
and sustaining at variable heights the more dense fluids, has been 

* Try the beer before a lecture, and immediately cork again. Some liquors 
lack too much the glutinous principle to make the experiment succeed. 



Give Experiment c, for showing the expansion of a gas by the bubbles rising 
from a fermenting liquid in an exhausted receiver. 



SUCTION PUMP. 



121 



Fig. 107. 






already illustrated ; it remains to speak of some of the pneu- 
matic machines employed in the raising of "vvater, etc. 

The Common Suction and Lifting 
Pump^ Fig. 107, is placed on a frame or 
platform. From the lower end of the 
pump-barrel a tube leads down into the 
well or cistern from which the water is to 
be pumped. In the piston is a clapper- 
. valve opening up ; another similar valve 

'\ is placed at the bottom of the barrel, also 
opening up. 

Experiment. — DraAV up the piston ; 
a partial vacuum will be formed in the 
barrel and tube below this. The pressure 
of the atmosphere on the water in the 
cistern will force this up the tube 
through the lower valve to fill the vacant 
space freed from air. Depress the pis- 
ton, the lower valve closes, while the 
^<^:^^^ upper opens and allows the water to pass 

above it. Raise it, its valve again closes, 
and the water above is lifted and flows out through the spout, 
and a second vacuum is formed, which causes the water from the 
well again to rise and follow the piston ; and so the operation 
continues so long as the piston is raised and loAvered as in the 
act of pumping. 

The piston seldom so fits the cylinder as to efiect a vacuum 
unless it be covered with water ; hence the necessity of pour- 
ing in some liquid at the first working of a new pump, or when 
the water has "run down."* The lower valve or "box" 



* This pump should be worked with a steady movement, and not by jerks. To 
start it, the top may be unscrewed and some water poured in. 



Why is water poured into the pump upon first starting it ? In the common 
Water-Pump why does the water rise in the barrel, and follow up the piston ? 
Explain the operation of the piston and valves in this. 



11 



122 



FORCE PUMP. 



must be placed within thirty feet of the water in the well, or 
this will not rise above it. 

100. The Fire- Engine. — The general action of this has been 
already shown (Experiment § 98). Fig. 108 shows more clearly 
the operation of the machme. On the form stands a plain 

Fig. 108. 



^ 




pump-cylinder, with a tube leading down into a reservoir of 
water. In this cylinder moves a solid plunger. As this 
plunger is drawn up, a vacuum is formed, and the liquid passes 
up through a valve at the bottom of the cylinder, as in the 
last experiment. Now depress the plunger ; the valve closes, 
and the liquid, having no other escape, is forced along a small 
tube leading from the bottom of the cylinder up through a 
drop-valve at the bottom of the condensing-chamber. From 
near the bottom of this chamber leads the hose, throuo;h which 



Explain the operation of the Fire-Engine, as shown in Fig. 108. 



THE SIPHOX. 



123 



Fig. 109. 



the water is forced and direction given to the stream at 
pleasure. 

Experiment. — Work the plunger ; the water is drawn up 
into the barrel and forced into the condensing-charnber. This 
compresses the air confined in this, causing it to react on the 
water, which is forced up through the tube and out of the hose 
in a continuous stream and to a considerable distance. The 
common fire-engine is provided with a pair of force-pumps 
with the piston-rods attached to a brake, so that while one is 
raising a column of water, the other, in descending, is forcing 
an equal quantity into the condensing-charnber: thus main- 
taining a larger and more efficient stream than bj the single 
plunger. 

101. Experwient. — The Siphon is 
an arrangement for causing water to flow 
over an elevated curve from one vessel 
to another. Arrange this, as seen in Fig. 
109. Apply the mouth to the suction- 
tube at the side, and, with the finger 
upon the lower extremity of the long 
arm, suck over the liquid. Remove the 
finger, and the fall of the water through 
the long arm will tend to produce a par- 
tial vacuum in the upper part, causing 
the water to rise in the short arm, and 
flow over the curve in a continuous 
stream. Liquors are thus transferred 
from one cask to another without disturbing their sediment. 

Intermittent springs^ which flow freely for a time and then 
cease, are explained on the principle of the Siphon. These 
springs are supplied from a reservoir in a hill or other eleva- 
tion, from which leads a siphon-shaped channel, opening out 




Explain the theory of its operation, as given in the experiment. With what 
is the common Fire-Engine provided '^ What is a Siphon ? For what is it 
commonly used ? On what principle do intermitting spi-ings flow ? 



124 



SIPUON FOUNTAIN. 



Fig. 110. 




Fig. 111. 



through the ground at a distance below. When 
the reservoir is j&lled so as to brmg the water 
above the siphon-curve, it will commence flowing, 
and continue so to do until the reservoir is 
emptied to a level with the mouth of the discharg- 
ing outlet, when it will cease. As the reservoir 
becomes filled above the curve, the spring will 
again commence to flow, 
and so alternate, at regular 
intervals. 

102. Experiment. — The 
Sijjhon Fountain^ Fig. 110, 
is an amusing modification 
of the same principle. This 
consists of a fountain formed 
by the flow of water fi^om 
the upper basin through the 
short arm up into the glass chamber. 
This flow is caused by the fall of the 
liquid through the long arm, whereby a 
vacuum is formed in the glass chamber 
causing the liquid to be forced up through 
a fine jet,* on the principle illustrated by 
Fig. 97. 

Experiment. — Hierds Fountain. 
— Pour water into the centre hole of 
the basin. Fig. Ill, until the upper 
chamber is nearly filled ; then screw into this centre hole a jet- 
tube, and fill the basin. The water will flow down through a 
side-tube, reaching nearly to the bottom of the lower chamber. 

* The hose of this jet should be quite small. 




Explain the cause of the irregular flow of such springs. Describe the Siphon 
Fountain and explain the cause of its flow. Explain the principle of the flow 
of Hiero's Fountain. 



HIERO'S FOUNTAIN. 125 

The air Tvill thus be compressed in this, and its elastic force 
comrQunicated by another tube leading from the upper part of 
the lower to the upper chamber, where it will react on the water 
in this chamber, and force it out through the opening, forming a 
beautiful ^'e^ cVeau. 



PNEUMATIC PROBLEMS. 

1. If the pressure of the atmosphere be 15 lbs. upon a square 
inch, what pressure will the body of an animal having a superficial 
area of 60 square feet sustain ? 

2. If a cubic inch of air weigh .29 of a grain, what weight of air 
will a globe holding 58 cubic inches contain ? 

3. An aeronaut, a few years since, made an ascension in a bal- 
loon to an elevation so great that his barometer stood at only 12.5 
inches ; supposing the same barometer to have stood at 30 inches at 
the level of the ocean, what portion of the atmosphere did he leave 
below him ? 

4. K the resistance of the air be 12 J pounds upon a square foot 
of the surface of a body moved through it with a velocity of 50 
miles per hour, what resistance will a rail-car, with a front 9 feet 
square, meet with, when moving with this velocity ? 

5. Supposing the density of the air to diminish in a geometrical 
series of ^ as the height above the level of the ocean increases in an 

K 1 14 21 ) 

arithmetical series of 7, thus, \ . , , > , what volume would a 



cubic mch of air at the level of the ocean occupy when raised to an 
elevation of 28 miles? 

6. What weight of air will be lifted at each stroke of a pump- 
handle where the upper side of the piston attached has an area of 16 
square inches ? 

11^ 



12(3 STEAM. 



THE MECHANICAL AGENCIES OE STEAM. 

1 03. Water, when heated to a temperature of 212°. under the 
oi-dinarj pressure of the atmosphere, is resolved into an invis- 
ible vapor, which, upon condensation by cold, becomes visible, 
and is known as steam. By such a change, in passing from a 
li(iuid to a vapor, the volume of the water is vastly increased, 
occupying a space about seventeen hundred times greater in 
the latter than in the former state. Such an increase of vol- 
ume gives to steam great power as a motive agent, which, through 
the steam-engine, has been applied of late years to propelling 
every variety of machinery. It is the province of chemistry 
to explain the laws of heat which regulate the formation of 
vapor or steam, and of natural philosophy to show the appli- 
cation of this as a mechanical force. 

The elastic force of vapor, from water at 212°, is just suffi- 
cient to overcome the pressure of the atmosphere, and at such 
a temperature in the open air ebullition accordingly goes on 
freely ; but, if water be confined in a tight vessel, its boiling- 
point will be raised (§ 92), and the density and elastic force of its 
vapor will increase with its temperature. This may be illustrated 
by an apparatus known as il/arce^' 5 St earn- Globe ^ Fig. 112. 

Experiment. — Unscrew from the brass globe the long her- 
metically sealed glass tube, with its graduated scale, 5, and 
through the opening pour in sufficient mercury, 7, to fill the 
bore of the tube, and then cover with water, 8, in the pro- 
portions shown in the cut ; replace the tube and scale. With 
the thermometer, 4, screwed into the side, and the stop-cock, 
6, open, apply heat. At 212° the water will boil. Now 

What is steam ? What is said of the volume of water in passing from a liquid 
to a vapor state ? What gives to steam its great motive power ? What is said of 
the elastic force of vapor from water at 212^ of Fahrenheit's thermometer ? 
Effect upon the boiling-point by heating water in a tight vessel ? Give the 
experiment with Marcet's Steam-Globe. Does the elastic force of the vapor 
increase with its temperature ? 



127 



close the stop-cock, and the boiling ceases, "while the tem- 
perature and elastic force of the steam increase. This, pressing 
on the water and mercury, will force the latter up the tube, at 



Fig. 112. 




heights varying with the tension and pressure 
of the steam. * Thus, at a temperature of 249°, 
the pressure of the vapor will have increased to 
that of two atmospheres, 293° to four: 
320° to six atmospheres, and so on. Thus, 
the elastic force corresponding to each ad- 
ditional degree of heat may be shown by 
this arrangement up to that of twelve or 
sixteen atmospheres. This force of vapor 
will be found, moreover, to increase in a far 
more rapid ratio than the temperature ; that 
is to say, a definite elevation of temperature 
produces a far greater increase of tension in 
the vapor at high, than at low temperatures. 
Thus, from 212° to 249° (an elevation of 
37°), the expansive force of the steam is in- 
creased only a single atmosphere ; while 
from 438° to 456° (a rise of only 18°), 
shows an additional increase in the elastic 
force of the vapor, equal io jive atmospheres, f 
104. Exferiment. — The elastic force of 
steam may be again illustrated by the Eolo- 
This is a hollow brass bulb, provided with a 
Heat the bulb over a spirit-lamp, and then 



'pile^ Fig. 113. 
handle and jet. 
plunge the jet and bulb in cold water ; the water will be driven 



* This tube has its upper end hermetically sealed, and the degree of pres- 
sure is shown as with the Condensing-Guage, Fig. 48. 

tWhen this steam-globe is charged to a pressure of seven, eight, or 
more atmospheres, attach to the stop-cock the air-gun^ revolving-jet, etc., 
and the same experiments may be performed by the elastic force of steam as 
by air, except that those by steam will be with flir more energy. 



Explain the experiment with the Eolopile, Fig. 113. 



128 



STEAM. 



Fig. 113. 



into the bulb to supply the partial vacuum occasioned bj the 
expulsion of the air when heated. When this is 
partly filled, hold it again in the flame. Steam will 
soon form from the water, and by its elastic force 
be driven forcibly through the jet.* 

The Steam- Engine. — This may be justly re- 
garded as the most valuable invention of modern 
times, since none have contributed more to the 
physical comforts and social delights of man. The 
present form of the steam-engine is due mainly to 
the genius and mechanical skill of Sir James Watt, 
of England, whose investigations on this subject 
commenced about the year 1763. The power of 
steam, as a propelling agent, was known for some 
time previous to this date, and applied through that 
form of engine now known as the Atmospheric- Engine. This was 
rn exceedingly rude and expensive application of steam, and it 
was from observing its defects that Mr. Watt 
was led to conceive the present form of 
the steam-engine. 

In order to appreciate the discovery of Watt, 
it is necessary to gain some idea of the oper- 
ation of the 

105. Atmospheric-Engine. — This may be 
illustrated by an apparatus shown in Fig. 114. 
To a straight brass cylinder is fitted a piston, 
with its rod moving through a hole in the 
screw-cap. To this cylinder is attached a 
wooden handle for holding, while heating, and 

* In place of the straight, attach the revolving jet, and this will be driven 
rapidly by the escaping steam. A steam-cannon, for firing balls in rapid 
succession, and to a great distance, has been devised. 

What is said in regard to the Steam-Engine ? To whom are we chiefly indebted 
for this invention ? By what form of engine was the power of steam first ap- 
plied ? "What is said of this engine ? Describe the Atmospheric-Engine ? 




STEAM-ENGINE. 129 

just above this, on the side of the cylinder, is a small spring- 
safety valve, and to the bottom of the same is soldered a copper 
globe for containing the water. 

Experiment. — Remove the screw-cap and pour in some water, 
or, which is better, alcohol ; replace, and hold the globe in the 
flame of a spirit-lamp ; steam will soon form, and by its expansive 
force drive up the piston ; then plunge the bulb in cold water. 
A vacuum will be suddenly formed by the condensation of the 
steam, and atmospheric pressure will force the piston to the 
bottom. Hold again in the flame, and then in the cold water, and 
again the piston will be driven up and down ; and so the process 
may be continued. If, now, the end of the piston-rod be attached 
to a wheel-crank or lever, the rise and fall of the piston will 
give motion to these, and afibrd a general illustration of the 
atmospheric-engine. 

Thus, it will be perceived, that, at every descent of the piston, 
the cylinder must be cooled, in order to condense the steam, and 
form the required vacuum. This was done by injecting into 
the cylinder a stream of cold water, which rendered a great ex- 
penditure of heat necessary to bring the water and cylinder 
again to the boiling-point of water. To remedy this. Watt con- 
ceived the idea of a separate condensation^ whereby a vacuum 
might be formed without at the same time cooling the cylinder. 
Accordingly, by a valve placed in the bottom of the cylinder, 
the steam was allowed to escape through a pipe into a separate 
chamber, where, by an injection of cold water, it was immediately 
condensed, and a vacuum thus formed in the cylinder. 

106. Again, tJie cooling of the cylinder^ by the entry of the 
air as the piston descended, was a second defect which engaged 
the attention of Watt. To remedy this he devised the plan of 
a close cylinder, employing steam instead of the atmosphere to 
force down the piston, thus keeping the cylinder continually 
heated by the steam, and so preventing the condensation of the 

Explain its operation by the figure. What plan did Watt conceive in order 
to avoid the cooling of the cylinder ? 



130 STEAM-ENGINE. 

same. A third difficulty now remained to be overcome, which 
was the removal of the air and warm water from the condenser. 
To effect this, he contrived to attach to the condenser a pump 
with a tube leading to the bottom of the chamber, and worked 
by steam, whereby these might be constantly pumped out. 

In addition to these important discoveries, was the providing 
a wood or metallic covering for the cylinder, whereby the escape 
of the heat and condensation of the steam was further prevented. 
This covering is known as the jacket. With these improvements 
an actual saving of three fourths of the fuel required for the 
atmospheric-engine was effected. With these preliminary re- 
marks, the learner is now prepared to understand the different 
parts, as shown in the 

107. Section Model of Watfs Improved Stearrt-Engine^ 
Fig. 115. — The cylinder, C, is made air-tight, so that the atmos- 
pheric air can exert no force on the piston moving in it. The steam 
from the boiler is conducted through the steam-pipe, and enters 
the cylinder alternately at A and B, being admitted through the 
slide-valves attached to the rod, D, which open and close these 
apertures at proper intervals, by means of an attachment con- 
necting with the eccentric, E, of the fly-wheel, F. From the 
steam-box leads the exhaust-pipe, which enters the loater-cistern 
or condenser, into which the steam, after performing its office in 
the cylinder, is conducted. Attached to this condenser is the 
air-pump, P, which is worked by a connection with the beam. 
A parallel work, H, connects the pump and piston-rods, and 
serves to keep these perpendicular and parallel during their 
ascent and descent. The beam-lever, which connects at one end 
with the piston-rod, is attached, and gives a crank motion to the 
fly-wheel, F (§37), at the other. In order to prevent irregu- 

How did he remedy the cooling of the cylinder by the entry of cold air as 
the piston descended ? What was Watt's third improvement in reference to 
the steam-engine ? What amount of fuel was saved by these improvements ? 
Point out the parts, and explain the operation of Watt's engine, by the section 
model, Fig. 115. 



STEAM-ENGINE. 



131 



larities in the force of the steam, and of course in the motion 
of the machinery driven, the governor, G (§38), is em- 
ployed.* 




Upon the boiler is placed a steam- valve for. the escape of the 
steam A\hen the boiler is overcharged, the pressure of which is 
regulated by the lever and weight. 

Owing to the accelerated motion given to the piston by the 
full force of the steam acting during its whole passage through 
the cylinder, it was found necessary in some way to regulate 

* The full construction of this regulator is not shown in the cut, and can 
only be comprehended by a working model. 



What remedy was devised in order to avoid the injury from the accelerated 
motion acquired by the piston ? 



132 STEAM- E.XGINE. 

this force. In order to do this, the valves attached to D (Fig. 
115) were so contrived as to close and cut ofi' the steam when the 
piston had made about one third of its descent, allowing this to 
drive the piston through the remaining two thirds, by its expan- 
sive force alone. Thus, a uniform velocity was given to the 
piston during its entire passage, besides effecting a great econ- 
omy in the expenditure of the steam. 

If the water-cistern or condenser, show^n in Fig. 115, be dis- 
pensed with, and the steam allowed to escape from the cylinder 
directly into the atmosphere, instead of passing into a vacuum, 
the piston will be resisted by the pressure of the atmosphere ; 
and of course, to obtain the same power for the engine as before, 
an additional tension of steam will be required equal to this 
pressure. Hence, owing to this additional force required, such 
are termed high-pressure engines. 

By dispensing with the condenser, air-pump, etc., the high- 
l)ressurc engine of the same power is much smaller, lighter, 
and cheaper. These are consequently much used in smaller 
steam-boats, and for locomotion on rail-roads. 

108. An Operative Model of the High-Pressure Horizontal 
Cylinder Engine^ is shown by Fig. 116. From what has 
been already said, the various parts of this engine may be read- 
ily understood by reference to the figure. 

B is the boiler ; C, the cylinder ; D, the steam-box, 
where the steam is admitted through the slide-valves to the 
cylinder ; E, the lever which opens the valve admitting the 
steam to the steam-box; F, the piston-rod, attached to the 
slide, R, which moves upon two parallel rods, and so guides 
the piston-rod in its motions to and fro. This corresponds to 
the parallel work of Watt's engine. G, the driving-rod, at- 
tached to the crank of the shaft, on which are fixed the balance- 



What will be the effect if the condenser be dispensed V\W\ ? What are high- 
pressure engines ? Some of the advantages of high-pressure over low-pressure 
engines? Where are high-pressure engines chiefly used? Point out the parts 
{y^ the higli-pressnre model, Fig. IIG. 



STEAM-EN GIXE. 



183 



wheels or regulators, 0, ; K, the pulley or drum, over 
which runs the belt connectmg with the machinery to be driven ; 




H, the eccentric, to which is attached the valve-rod leadmg 
to' the steam-box; M, the lever of the safety-valve; Q, the 
eduction-pipe leading from the cylinder and entering the open 
cistern, P, where it is partly condensed, and forced agam into 
the boiler by meaiis oi" the force-pump, J. 
12 



134 



STEAM-ENGINE. 



Fig. 117 presents a Working Model of the Upright Cyl'm- 
der High-Pressure Engine. The parts of this maj be learned 



Fig. 117. 




far more rearlilj by witnessing its operation, than from any 
written description. 

i?ewar A:. —Water freed from air will not boil at L'12- ; if the access of air 
to such be prevented, and the temperature be raised to 270^ and above, tlic 
whole mass of liquid may become highly explosive, exploding with the force of 
gunpowder. Experiment shows this to be especially true when a very small 
quantity of common water is suffered to enter the heated liquid. This is 
thought in many instances to be the cause of the fearful explosia-^s of steam- 
boilers, —the water of the boiler being freed from its air by ebullition, and the 
feed- water admitted when this is at a high temperature, causes the whole to 
explode with a force far exceeding that of steam at the pressure indicated. 

Various attempts have been made of late to substitute hot air, ether, vapor, 
etc., for the steam from water, but as yet without success. The bi-sulphuret 
of carbon, however, is said to have been recently used, and the experiment 
to promise success. (See Scientific American, June 30, 1855 ) 



What docs figure 1 1 7 represent ? 



WINDS. 135 



METEOROLOGY. 

109. Meteorology treats of the various ^phenomena of the 
Atmosphere. — The atmosphere which envelopes the earth may 
be regarded as a vast laboratory, in which nature, through the 
agencies of heat and moisture, is working constant changes. 
Among the more important results of these atmospheric changes 
may be mentioned the formation of winds, clouds, rain, hail, 
dew, frosts, etc. 

Winds are caused by disturbances in the equilibrium of 
the at'inosphere^ produced by heat from, the sun and other 
sources. — Whenever any portion of the earth's atmosphere 
becomes heated, a rarefaction is the result. This causes the 
cooler and denser air from the surrounding portions to flow in 
to this rarefied space, and winds blowing from different quarters 
are the result. Examples of this are furnished by the sea and 
land breezes of islands situated in the midst of the ocean. 

As the earth receives and imparts the heat derived from the 
sun far more readily than the water does, the atmosphere 
incumbent upon the former becomes more heated and rarefied 
during the day : this causes the air over the land to ascend, 
while the cooler and denser portions from the ocean flow in on 
all sides to fill the partial void ; and, hence, the cool sea-breeze 
that is usually felt in such situations during the middle of a 
summer's day. 

As the sun declines at evening, the land, which parts more 
readily with its heat, soon becomes cooler than the water. The 
atmosphere resting over it, also partaking of the change, tfie sea- 
breeze gradually dies away ; during the night the aerial currents 
are reversed, and blow from the land towards the ocean, causing 
the land-breeze of the mornino;. With the heat from the ascendino; 



Of what does Meteorology treat ? The more important results of the changes 
which take place in the atmosphere ? How are winds produced ? Give an 
explanation of the causes of the sea and land breezes. 



136 WINDS. 

surij a sea-breeze is again formed ; and so the change is repeated. 
These sea and land breezes are more marked in tropical lati- 
tudes, where the heat of the sun is more intense. 

They are also experienced to a certain extent for a few miles 
inland, along the main coast bordering upon the ocean, and 
afford an agreeable relief from the heat of the summer's sun. 

The same cause of winds is sometimes illustrated, on a less 
extended scale, in the burning of buildings during a calm 
evening, when a gentle breeze may be perceived setting in on 
all sides towards the heated space. 

The violent winds frequently attending powerful rain-storms 
are supposed to be produced by the rapid condensation of the 
vapor of the atmosphere in the form of rain. The great re- 
duction in volume, caused by such a condensation of vapor, 
produces a sudden rarefaction of the air, which causes the sur- 
rounding portions to rush in with violence to fill the rarefied 
space, thus giving rise to the terrific winds often experienced 
during such storms, especially in tropical latitudes. 

110. Whiiiwiads are caused by the meeting of winds from dif- 
ferent quarters so as to produce a gyratory or whirling movement 
of the air. The tendency of the central portion of these is 
usually upwards, as seen by the course of the light bodies taken 
up by such in their passage across an open field. In tropical 
regions these whirlwinds are often extremely violent, consti- 
tuting hurricanes and tornadoes.* When these whirlwinds 

* The terrific force of the wind in tornadoes was seen a few years since in one 
which p-issed over and devastated a portion of the island of Guadaloupe. 
Houses firmly built were demolished ; cannons were hurled from the top of the 
pai'apets of the batteries on which they were planted ; a thick plank, three 
feet in length and eight inches in breadth, was driven with such a force 
through the air that it perforated a palm-tree about seventeen inches in diam- 
eter, through and through. 

How is the cause of winds shown in the burning of buildings during a calm 
evening ? Cause of the violent winds often accompanying i-ain-storms ? How 
are whirlwinds produced? What is said of these in tropical regions? 



MISTS AND CLOUDS. 137 

pass over bodies of water, if sufficiently violent, they produce 
ivatei^-sjjouts. 

Trade Winds. — These are winds which prevail in tropical 
latitudes, and usually extend to a distance of about 28° or 30° 
on each side of the equator. Their direction is from the north- 
east towards the south- vfcst on the north side of the equator, 
and from the south-east towards the north-west on the south 
side of the equator, meeting and merging at the equator into 
a direct east wind. The direction of these winds is uniform 
throughout the whole year, except where certain local causes 
interfere. 

The trade winds are the result of two causes combined, 
namely, the rarefaction of the air of the torrid zone, produced 
by the heat of the sun, which causes the colder air of higher 
latitudes to flow in towards the equator, and the rotary motion 
of the earth on its axis from west to east, causing these winds 
to fall behind, or have a tendency towards the west, as they 
approach the equator, where the velocity of the earth's surface 
becomes greater. The trade winds were so called because of 
the facilities which they afford to vessels engaged in trade and 
commerce. 

111. Mists or clouds are formed from the watery vapor of the 
atmosphere condensed by cold so as to become visible. Atmos- 
pheric ail' is capable of taking up and holding in solution a 
great amount of vapor. This becomes visible only when the 
air in which it is dissolved is cooled to a certain point, when 
it assumes the form of minute vesicles or floating bubbles, and 
appears as a mist or cloud. 

The quantity of watery vapor which a given volume of air 
will hold in solution, depends on its temperature, increasing 
with this, but not in the same ratio. If a body of air at a 

Where do the trade winds prevail, and what is the direction of their course ? 
Two causes of these? Why called trade winds ? From what are clouds formed ? 
What is said of atmospheric air in its relations to watery vapor ? On what 
does the quantity of vapor which a given volume of air will hold depend ? 

12* 



138 MISTS AND CLOUDS. 

high temperature, say 72°, have this increased ten degrees, its 
capacity for holding vapor will become far greater by such in- 
crease, than if its temperature be increased ten degrees when 
at 32°. Accordingly, if two equal bodies of air, one at 80°, 
and the other at 40°, when saturated with vapor, meet and com- 
mingle, their mean capacity for vapor will not be the same as 
their mean temperature, 60°, but considerably above this ; so 
that a portion of their vapor will be condensed, and become 
visible as a mist. 

In this manner clouds are supposed to be formed when 
opposite currents of air, of different temperatures, and saturated 
with watery vapor, meet. 

"When a warm current of air, charged with vapor, blows over 
a cooler surface of land or water, the vapor of the air becomes 
condensed, and appears as a fog or mist. So, also, when the 
water is warmer than the incumbent atmosphere, the vapors 
rising from it will be in like manner condensed. Thus, the 
warm currents of the Gulf-stream, meeting with the cooler air 
of the northern latitudes, give rise to the dense fogs which 
prevail so extensively upon the banks of Newfoundland. 

The cause of the rapid formation of thunder-clouds, during 
a summer's day, is generally due to the vapors carried up by 
the warm currents rising from the earth's surface, and which 
are condensed by the cold of the upper regions. Storms are 
usually formed by the meeting of currents of air, of different 
temperatures, blowing from opposite quarters, and charged with 
vapors. Clouds which form in elevated regions oftentimes disap- 
pear, or are dissolved by sinking down into the warmer air below, 
which is capable of holding the vapor of these in an invisible 
solution. 

Explain the manner in which vapor may be condensed and become visible 
by the meeting of two opposite currents of air of different temperatures. Ex- 
plain the formation of fogs resting down upon the surface of land or water. 
How are the fogs upon the banks of Newfoundland produced ? 



RAIN AND HAIL. 139 

112. Rain. — "When the watery vesicles of a cloud unite and 
become too heavy to be longer supported by the air, they 
descend in drops of rain. The quantity of rain falling in a 
given time depends on the rapidity with which the vapor of the 
cloud is condensed. In tropical latitudes, where the causes 
combine to produce a rapid condensation of the vapor, the quan- 
tity of rain which falls in a short space of time is often sur- 
prisingly great.* 

Hail is simply drops of rain frozen by the cold of the elevated 
regions. Hail-stones, weighing several ounces, occasionally fall, 
doing serious injury to dwellings as well as vegetation. How 
these can be sustained in the atmosphere a sufficient time to 
allow of such formations, has been a question among meteor- 
ologists. The cause is, however, usually attributed to whirl- 
winds, by which a quantity of dense vapor is suddenly trans- 
ported into the colder regions above, and there supported for a 
sufficient time to allow of their formation. Hail-storms are 
confined to the temperate zone, and the time of their duration 
seldom exceeds fifteen or twenty minutes. 

113. Dew is formed by vapor deposited from the air in 
contact with cold surfaces. — Thus, if a tumbler of cold water 
be placed upon the table, in a summer day, its outside surface 
usually becomes covered with moisture deposited from the con- 
tiguous air. The temperature at which the air will deposit this 

* At Bombay there fell in one day six inches of rain ; at Cayenne, ten 
inches fell in ten hours. The annual quantity of rain which falls at London 
is about twenty-five inches ; at Paris, twenty inches ; while that which falls 
in a single year on the coast of Malabar sometimes reaches one hundred and 
twenty- three inches, and at St. Domingo, one hundred and fitly inches. Dur- 
ing the fall of the torrents of rain in tropical latitudes, the drops are often 
surprisingly large, and fall with a force sufficient to cause pain where they 
strike the unprotected surface of the body. 

. How are rain-drops produced ? What is said of rain in tropical latitudes ? 
What is hail ? How are hail-stones of a large size probably formed ? Where 
do hail-storms prevail ? How is dew formed ? Illustrate the formation of dew 
in the case of a tumbler of water. What is the dew-point? 



140 DEW. 

moisture, or dew, is called the deiv-polnt. This varies at 
different times, according to the amount of vapor with which the 
air is charged. Bj placing a thermometer in a tumbler of 
water, and noting the temperature at which the vapor begins 
to deposit, the dew-point, and, consequently, the proportional 
amount of watery vapor in the air, may be at any time ascer- 
tained. 

The dew formed on leaves and vegetables during a calm, 
clear night, is caused by the readiness with which such objects 
part with their heat and become cooled in the absence of the 
sun. Bodies, such as stones, sand, etc., which retain their 
heat, or lose it less readily, seldom become cooled, during the 
night, to a temperature sufficiently low for a dew deposit ; and 
hence, the usual freedom of these from dew.* Clouds suspended 
over the earth radiate back the heat which they receive from 
this, and thus prevent the temperature from falling ; for this 
reason dew seldom forms during a cloudy night. From the 
same cause, straw, etc., spread over vegetables, prevent the form- 
ation of dew on these. So winds, which serve to change the 
layers of air, and bring warm currents of this in contact with 
the objects on the earth's surface, prevent the formation of dew. 
Hoar-frost is frozen dew. 

* The nice discrimination observed in the deposit of dew on various objects 
may be adduced as one of the numberless proofs of an intelligent, designing 
Author of Nature. Thus, we see this fertilizing element gather copiously on 
vegetables, which most require its reviving influence ; while bodies of water, 
stones, sands, and barren wastes, where a dewy deposit would be useless, re- 
ceive comparatively little moisture from this source. In regions where rain 
seldom falls, vegetation is in many instances sustained by the copious dew de- 
posited during the night : this is seen in Palestine, and other oriental countries ; 
also in Chili, and along the western coasts of America. 



Why does dew form on vegetables rather than on stones, sand, etc.? How 
do clouds prevent the formation of dew ? Why does this not form in windy 
nights? What is hoar-frost ? 



CAUSE OF SOUND. 141 



SOUND. 



114. When a sounding body, as a bell, for instance, is struck, 
it assumes a tremulous or vibratory motion : this imparts to the 
surrounding air a series of oscillations, which, impinging on 
the membrane of the drum of the ear, excite in this also trem- 
ulous motions ; through the medium of the included air, or the 
delicate chain of bones connecting with the labyrinth of the ear, 
these motions are communicated to the fluid which fills this 
labyrinth, which, acting on the auditory nerve, produces the 
sensation of sound. 

Air, as we have already shown, is an exceedingly elastic 
fluid; and, as elastic bodies are in general the mediums of 
sound, this becomes the diief vehicle for communicating to the 
ear the vibrations from sounding bodies. 

Aerial oscillations, or sound-waves, are produced in, and 
traverse the air, in a manner similar to the undulations of water 
caused by throwing into it a pebble. Thus, if a stone be thrown 
into a smooth pond, a concussion is produced which imparts to 
the water a series of oscillations or waves ; each layer of water 
transmitting the vibratory motion it receives, in a somewhat 
diminished degree, to the next, and so spreading in every direc- 
tion from the disturbing cause, until the whole surface of the 
pond is agitated, or the tremulous motion dies away in the dis- 
tance. So, when a sounding body is struck, a like undulatory 
movement is produced in the air, which is transmitted, in a 
manner analogous to that of water, except with far greater 
fiicility, causing, as before stated, a sound. 

When these vibrations, excited in the air by a sounding body, 
are uniform and regular, as where a harp-string is struck by 

Explain the process by which sound is pi-oduced. Why is air the chief 
vehicle of sound ? How do the waves of souud traverse the air ? Give an illus- 
tration in the case of water. 



142 



CAUSE OF SOUND. 



Fig. 118. 



the finger, a perfect sound, or tone^ is produced; but if the 
vibrations take place irregularly, and are not isochronous,* as 
in the explosion of a gun, a noise alone is the result. 

115. Air is, in general, the medium of sound, although 
nearly all other elastic bodies are capable of conducting it more 
or less perfectly. Thus, a sonorous body, properly insulated, 
becomes inaudible in a space from which the air has been 
removed. This may be shown as follows : 

Experiment. — The Bellin vacuo, Fig. 118, is abell insulated, 
as far as possible, by suspending from 
a loosely-twisted silk cord placed 
in a vacuum. Screw the post into 
the centre hole of the pump-plate ; 
press down the sliding-rod, and at- 
tach its string to the handle of the 
bell, and ^en cover with the glass, 
drawdng up the sliding-rod at the 
same time to the position shown in 
the figure. After placing a drop 
of oil on the sliding-rod, and seeing 
that the bell-glass is well fitted to 
the plate (§ 66), ring the bell by 
means of the rod. The air, which 
now fills the receiver and surrounds 
the bell, will serve as a medium for 
transmitting its vibrations, and the 
ringing will be clearly heard. Now 
perfect a vacuum in the bell-glass, 
and again ring the bell as before. This has now become inau- 
dible, since the medium through which its vibrations are con- 
ducted has been removed by the exhaustion. On gradually 




* Isos, equal ; chronos, time. 



How is a perfect sound or tone caused? How a mere noise? Give the 
experiment of the Bell in vacuo. 



CAUSE OF SOUND. 143 

admitting the air, the tone becomes louder and louder, mth the 
increasing density of the surrounding air. 

Sound increases with the density of the air transmitting the 
vibrations of the sonorous body. This may be illustrated by 
the following 

Experiment a. — Suspend a bell, by means of a small rubber 
tube prepared for the purpose, in the glass condensing-chamber, 
Fig. 80. With the stop-cock closed, ring the bell, by giving 
motion to the chamber, and mark the intensity of the sound ; 
then, with the condenser, force in three or four atmospheres, 
and ring again as before : the tone will now be perceived to 
have increased in loudness, with the increased density of the air 
surrounding the bell.* 

Thus, in ascending high mountains, or rising in balloons, the 
intensity of sound is found to diminish as the rarity of the air 
increases. Saussure found that on the summit of Mont Blanc 
the explosion of a pistol appeared no louder than that of a 
cracker at the surface of the ocean : and aeronauts have often 
noticed a great diminution in the intensity of the voice at their 
greatest elevations. On the contrary, the condensed air of the 
diving-bell (§ 78), when lowered to a great depth in water, is 
found to conduct sound with such facility as to render even the 
slightest whisper painful to the ear. 

116. The conducting poirer of air varies with its uniform- 
ity^ its density, and humidity. — Thus, in a calm, frosty morn- 
ing, before the uniform density of the atmosphere has become 
disturbed by the sun's heat, the sound of the voice may be often 
heard to a surprising distance, especially over a level surface, 
as a body of water. Under such circumstances, conversation 

* By means of a hose connecting this chamber with the air-pump, it may be 
exhausted, and the bell in vacuo also illustrated. 

What is said of the density of air in reference to sound ? Give the experi- 
ment for showing that sound increases with the density of the air. What is 
said of sound on high elevations ? What is said of sound in the diving-bell ? 
With what does the conducting power of air vary? Give the illustrations. 



144 SOLIDS CONDUCT SOUND. 

has been carried on across an intervening space of nearly two 
miles. The watchword, AlVs well! has been heard from Old 
to New Gibraltar, a distance of ten or twelve miles ; * w^hile 
the sound of a cannonading at sea is said to have been conveyed 
to a distance of two hundred miles. Sounds are also peculiarly 
clear and loud in a humid atmosphere, such as usually precedes 
a storm. • This, however, is not attributable, as is sometimes 
supposed, to an increased density of the air, but rather to the 
superior facility of the watery particles for conducting sound. 

117. Solids as well as liquids are in general better con- 
ductors of sound than air. — Thus, the scratch of an aAvl at one 
end of a series of pump-logs properly joined, w^ill be distinctly 
heard by the ear, applied at the other, although scarcely audible 
through the air, to the person making it. So, by resting the 
ear upon the iron rail of a railroad, the approach of a train of 
cars, or the strokes from the hammer of a workman, may be 
heard for miles. In this manner the conducting power of the 
earth enables the American Indian to distinguish, at a surpris- 
ing distance, the tramp of buifaloes, or the proximity of an 
enemy. 

It is easy to ascertain w^hether a kettle boils, by placing one 
end of a stick, or poker, on the lid, and the other end to the 
ear ; the bubbling of the water then appears as loud as the 
rattling of a carriage in the street. A slight blow given to a 
steel poker, or triangle, of which one end is held to the car, 
produces a sound even painfully loud. Two persons stopping 
their ears and holding a stick between their teeth, may hold 
conversation through the conducting power of the stick and the 
solids of the body. 

The fact of solids conveying sounds more readily than aii- 

* Olmsted's Philosophy. 



What is said of a humid atmosphere for conducting sound ? To what is this 
attributed? What is said of the power of solids and liquids for conductiii;. 
sound ? Give illustrations of the conducting power of solids. 



LIQUIDS CONDUCT SOUND. 145 

led to the discovery of the Stethoscope. This instrument con- 
sists of a wooden cylinder, which is applied to the surface of the 
body, over any internal organs, as the lungs, for instance, and 
thus, by sounds conveyed through this to the practised ear, the 
healthy or diseased condition of these internal organs is deter- 
mined with nearly the same accuracy as though seen by the 
eye itself =^ 

The superior conducting power of solids over air was deter- 
mined by a series of experiments performed by M. Biot, who 
availed himself of the laying of a train of iron pipes arranged 
for conveying water into Paris. By suspending a bell in the 
cavity, at one end of the series, so that a hammer should strike 
this and the inside of the pipe at the same instant, the compara- 
tive power of the air and metal for conducting sound was 
determined. 

By these experiments it was ascertained that cast-iron con- 
ducts sound about ten times as rapidly as air. 

118. Liquids are also good vehicles for soimd. — Thus, if the 
head be held beneath the surface of the water, and a person upon 
the opposite side of a pond, a half a mile or more distant, strike 
together two stones in the water, the sound of the concussion 
will be distinctly conveyed to the ear through the water, although 
through the air the same would be audible but a few feet. M. 
Colladon heard the sound of a bell, struck under water, across 
the whole breadth of Lake Geneva, a distance of nine miles ; 
this appeared to be conveyed through the water with about four 
times the velocity that sound ordinarily traverses the air. 

119. The velocity of sound is progressive^ and in air, at the 
surface of the ocean, and at a temperature of 62°, has been ascer- 

* Dr. Arnott. 

Principle on -wliich the Stethoscope acts ? Describe this and its uses. State 
the experiments of M. Blot. How much more rapidly does iron conduct sound 
than air ? What is said of the conducting power of liquids ? Give an illus- 
tration. What is said of the velocity of sound, and the distance it moves in a 
second ? 

13 



146 VELOCITY OF SOUND. 

tained by careful experiments, to be very nearly 1125 feet per 
second. Hence, by knowing the rate at which sound travels, 
the distance of a sounding body, as a cannon for instance, may 
be easily determined, and with a good degree of accuracy. 

This is ascertained by multiplying the number of seconds 
which elapse after the flash before the report or thunder is 
heard, by 1125, and then reducing this product to miles, by 
dividing by 5280, the number of feet in a mile. Thus, if a 
flash of lightning is seen fifteen seconds before the report is 
heard, 1125 X 15 -r- 5280, gives very nearly 3.19 miles, the 
distance of the discharge.* 

The marching of -a long procession to music affords a good 
example of the progressive nature of sound. Thus, while each 
platoon keeps step to the music, as it is heard, a gradual varia- 
tion will be seen along the whole line, those in the rear, and 
most remote from the music, may be seen, perhaps, an entire 
beat behind those in the front ranks. 

120. Reflection of so2ind. — "When the vibrations or sound- 
waves excited in air, impinge against any plane solid surface, they 
are reflected or thrown back the same as when the waves of water 
strike against a wall or other body ; this reflection, or return 
of sound to the ear, occasions an echo. Sound-waves follow the 
same laws of reflection which govern more ponderable matter, 
and are always reflected at the same angle at which they strike 
a surface ; so that, to hear an echo of his own voice, the person 
must stand dii'ectly before the surface from which it is last 
reflected. 

* Since light moves with an immense velocity (200,000 miles per second, 
nearly), for any distance on the earth's surface, it is regarded as instanta- 



How may the distance of a sounding body, as a cannon for instance, be de- 
termined ? Give an illustration of the progressive movement of sound in the 
marching of a long procession What is said in regard to the reflection of 
Bound ? What is an echo ? 



ECHO. — SPEAKING-TRUMPET. 147 

The quickness with which an echo is returned depends, of 
course, on the distance, so that an object situated at half the 
distance which sound traverses in a second, say five hundred 
and sixty-two feet, would return a sound, to the person origin- 
ating it, in just one second ; so that if the reflecting surface be 
sufficiently remote, several syllables or a short sentence may 
be uttered, and receive a distinct return. At Lurleyfels, on the 
Ehine, there is an echo which repeats seventeen times. 

The echo of the Capo di Bouve, as well as that of the 
Metelli at Rome, w^as celebrated among the ancients. It is a 
matter of tradition that the latter was capable of repeating the 
first line of the iEneid, which contains fifteen syllables, eight 
times distinctly. An echo in a building at Pavia is said to 
have answered a question by repeating its last syllable thirty 
times. 

Sound is reflected from concave surfaces, and collected the 
same as light and heat ; hence, beneath domes and arched ceil- 
ings, the sound of the voice becomes often quite painful. A 
notorious instance of a sound-collecting surface was the ear of 
Dionysiiis^ m the dungeons of Syracuse. The roof of the 
prison was so formed as to collect the words and even the 
whispers of the unhappy prisoners, and to direct them along a 
hidden conduit to where the tyrant sat listening. The wide- 
spread sail of a ship, rendered concave by a gentle breeze, is 
also a good collector of sound. By this means the sound of 
bells rung at St. Salvador was heard on the deck of a vessel one 
hundred miles distant.* 

121. The Sjjeakiiig- Trumpet. — This is an instrument em- 
ployed by commanders of vessels, generals of armies, and 

*Arnott's Physics. 



Upon ■what do the number of echoes from a surface depend ? What illus- 
trations are given ? What is said of sound reflected from concave surfaces ? 
Example in case of the ear of Dionysius ? The sail of a ship ? By whom and 
■where is the Speaking-Trumpet used ? 




148 MUSICAL SOUNDS. 

others, for transmitting their orders during the noise of the 
tempest, or the din of battle. The rays of sound, proceeding 
from the mouth when applied to the trumpet, instead of diverg- 
ing and being scattered tln^ough the surrounding atmosphere, 

are reflected from the 
^'^' ^^^' sides of this instrument, 

and conducted forward 
in straight lines, thus 
giving great additional 
power to the voice. The 
course of the rays of 
sound, proceeding from 
the mouth through this instrument, may be shown by Fig. 
119, which exhibits a common form of the speaking-trumpet. 

122. Musical Sotmds. — Sound, as we have already re- 
marked, is the result of vibrations excited in air or other elastic 
media by a sounding body. When these vibrations of the sound- 
ing body occur at sufficient intervals, so as to be seen by the 
eye, the impulses given to the air will be attended to separately 
by the ear, and a noise only will be produced ; but, if these be 
repeated with sufficient frequency, the ear wdll be unable to 
distinguish the separate vibrations, and a continuous sound or 
to7ie will be heard. Thus, if the string of a bass-viol be suffi- 
ciently slack, its vibrations may be seen, and only a harsh, dis- 
agreeable sound will be produced ; but, as the tension of this 
is increased, its vibrations become more rapid, until it gives 
forth a smooth and agreeable tone. 

The 2? itch of a musical chord depends on the number of vibra- 
tions it makes in a given time, and these vary with the length, 
the diameter, and the tension of the chord. Thus, with a given 
diameter and length, the greater the tension the more frequent 



How does this instrument aid the voice in transmitting commands ? What 
does Fig. 119 show ? How are Musical Sounds produced ? Illustration? On 
what does the pitch of a musical chord depend ? 



MUSICAL SCALE. 149 

will be the vibrations, and consequently the higher the pitch or 
tone. And, again, with a given length and degree of tension, 
the less the diameter of the chord the higher will be its tone. 
While, again, with a given diameter and tension, the less the 
lengthy the higher the tone of the musical chord. 

These truths are familiarly illustrated in the violin. The 
low or base string is thick and heavy from being covered with 
fine wire, while the others gradually diminish in size and 
weight up to the smallest or treble. These strings are tuned 
to each other by being attached at one end to movable pins, 
which, when turned, increase or diminish their tension. The 
sound then produced by each may be varied to a certain extent 
by the performers pressing the string at different points with 
the fingers, so as to vary the length of the vibrating portion.* 

When a musical tone is produced by a definite number of 
vibrations of the sounding body, it is termed a note. A col- 
lection of eight consecutive notes forms an octave ; and one 
octave is said to be higher or lower than another when the 
notes it contains are produced by a greater or smaller number 
of vibrations in a given time. Thus, if a particular note of any 
octave be produced by a given rate of vibration, the vibrations 
producing the corresponding note of the next octave below will 
be one half, and of the next above, twice as rapid. These 
notes constitute the diatonic scale or gamut. The English 
names for these, as well as the ratio of vibrations corresponding 
to each, are as follows : 

CDEFGABC 
1 f -I- I f t ¥- 2 

Thus, these eight notes constitute the scale or steps by 
which the voice naturally ascends from any tone to the corre- 

* Arnott. 

How do vibrations of such a chord vary ? Give an illustration in case of the 
violin. What is a note in music ? An octave ? When is one octave said to be 
higher or lower than another ? Why are these eight notes called the scale ? 

13* 



150 WIND INSTRUMENTS. 

ponding tone above produced bj vibrations twice as rapid ; and 
however far this musical scale be extended, it will still be found 
but a repetition of similar octaves. 

123. Wind Instruments^ as the flute, the organ, etc., emit 
sounds from the longitudinal vibrations of the column of air con- 
fined within their tubes. As with musical strings, these vibrations 
vary according to the length of the tube, being more frequent 
as these are shorter. When one end of the tube is closed, the 
note is rendered twice as grave, since the sound-wave has to 
return after passing in. The length of the air ^dbrations and 
pitch of the tone are regulated by the opening and closing of 
holes arranged along the side of the tube, as in the pipe and flute. 

Musical tones, by whatever instrument produced, have 
to each other the same numerical relations as the vibrations 
which constitute them. The different qualities of tone, there- 
fore, from different instruments, can only depend on the pecu- 
liarities of the single vibration, as to whether they are sharp or 
soft, strong or weak. 

The cultivation of no art exerts a more refined and human- 
izing influence than that of music. The simple, yet expressive 
songs of the school-room and the nursery shed over the mind 
and character an influence often felt through all the devious 
course of subsequent life. How often does the sound of some 
favorite air, learned during the agreeable recreations of innocent 
childhood, revive all that was dear in home and kindred, and 
point the wanderer back to duty ! Music is the language of 
nature* intelligible at once to all susceptible minds, and, in 
a degree, even to the inferior animals. Yet the love of novelty 
and fashion among professors in the art often throws over it so 
many ornaments and accompaniments, that the melody^ in which 

AVhat is said of octaves above or below a given octave ? What is said of the 
vibrations in wind-instruments, as the flute, etc. ? Of the relations of the 
vibrations which produce musical tones ? What is said in relation to the in- 
fluence of music ? Of the fashionable tricks of the voice by some professors of 
the musical art ? 



VENTRILOQUISM. 151 

the idea and sentiment really reside, is masked and lost. Some 
of the tricks on the voice and on instruments, at present so 
common, are to natural or graceful music what tumj^ling and 
rope-dancing are to natural and graceful gesture. 

124. No instrument is capable of producing sounds of greater 
variety and melody than that which forms the human voice. 
This consists of two delicate membranes situated at the top of 
the wind-pipe, with a slit or opening, called the glottis^ left 
between them for the passage of the air. By varying the 
tension of these membranes, and the size of the opening, the 
innumerable variety of tones of which the voice is capable is 
produced. 

The human voice has been in some instances so trained as to 
be capable of imitating with a wonderful degree of accuracy the 
various instruments of a musical band, as the bugle, the clar- 
inet, etc. A band of twelve Germans, a few years since, per- 
formed in some of the European cities a variety of difficult airs, 
as waltzes, polkas, etc., imitating, with their voices alone, as 
many different musical instruments. 

The art of Ve?ii?'iloqids?n * consists in the power of the per- 
former so to modulate and vary his voice as to imitate differ- 
ent sounds at varying distances, and thus to deceive the judg- 
ment of the listener in regard to the direction from whence 
these proceed. Thus, the sound of voices apparently at a 
distance, as, for instance, of two disputants engaged in angry 
debate, may be created by the operator, so as to appear to the 
bystanders as a reality. 

The art of imitating sounds, as seen in the case of the ven- 
triloquist, is in a great measure the result of careful practice. 
Thus, almost any person possessed of a good ear for sounds 

* Ventre, the belly, and loquor, to speak. 

"What is said of the organs or instrument by which the human voice is 
formed ? Of what does this instrument consist ? In what does the art of Ven- 
triloquism consist ? How are these illusions of the voice produced ? 



152 THE EAR. 

may, by habitual practice, become able to imitate the varying 
tones of voice of others, and the sounds of different animals, 
with great precision. 

The inechanism of the ear^ whereby sound is produced, and 
the sensation conveyed to the brain, is truly wonderful. This, 
in the human species, consists of three distinct parts. The ex- 
ternal and visible part of the ear serves, like the mouth of an 
ear-trumpet, to collect the rays of sound, and reflect them in- 
wards, through a gradually contracting tube, to an aperture 
within the skull, covered by a delicate membrane tightly 
stretched over it. Behind this membrane is a chamber, filled 
with air, known as the tyinpanuni^ or drum of the ear. Thus, 
the least concussion of the air without causes this membrane 
to vibrate and produce the sensation of sound. 

From this chamber of the drum leads the eustachean tube, 
opening into the mouth, and so forming a free communication 
between it and the external air, whereby an equilibrium of 
pressure is constantly maintained. Whenever this tube, from 
any cause, becomes filled so as to interrupt the free communi- 
cation with the external air, a humming sound or ringing in 
the head is produced. At the inner extremity of the chamber 
just mentioned is a second opening, called the fenestra ovalis^ 
covered by a second elastic membrane, w^hich completes the 
analogy of the drum. Various other delicate appendages serve 
to complete the apparatus of this wonderful organ. 



PEACTICAL PROBLEMS. 

1. If the pressui'e of steam upon the boiler of a locomotive bo 
65 lbs. against a square inch, what would be the entire force of this 
acting against a surface of 60 square feet ? 

What is said of the art of imitating sounds ? Of how many parts does the 
ear consist? Use of the external part of the ear? Describe the tympanum. 
Cause of tlie humming sound sometimes heard ? 



PROBLEMS. 153 

2. In such a locomotive, what would be the amount of pressure 
against a piston 12 inches in diameter ? 

3. If the evaporation from a square foot of ground was found in 
12 hours to be 2 gills, how many gallons would evaporate from a 
square acre in the same time ? 

4. The discharge of a gun from a frigate was seen 28 seconds 
before a report was heard ; allowing sound to travel 1,125 feet per 
second, how far distant was the frigate ? 

5. A flash of lightning was seen 13 seconds before the thunder 
was heard ; what was its distance ? 

6. A person saw the flash of a cannon fired from a ship 4 miles 
distant ; how long after, if at all, did he hear a report ? 

7. How long after a sudden shout will an echo be returned from 
a cliff 80 rods distant ? 

8. If a gun be fired, and the sound returned from a wood in 2| 
seconds, how far distant is the wood ? 

9. Wishing to ascertain the depth of a cavern which could not be 
descended into, a stone was dropped and seen to strike some water at 
the bottom two seconds before a report was heard ; how deep was the 
cavern ? 

10. In the bombardment of a certain fortress a shell fired from a 
frigate was seen to enter and instantly explode the magazine, a report 
of which reached the frigate in 10 seconds after the explosion; what 
was the distance the shell was fired ? 



154 



THE MAGNET. 



MAGNETISM. 




125. The remarkable property, possessed by certain fer- 
ruginous ores, of attracting iron, was 
well known to the ancients. These ores 
vere termed Magnets^ from Mag?iesia, 
L town of Lydia, where they were said 
to abound. 

This natural magnet, or, as it is gen- 
erally termed, lodestone, has the power 
of producing similar magnetic qualities 
in iron and steel, when brought in con- 
tact with it, and rendering them also 
magnetic. These latter are therefore 

T^''(lffV //v:v,^V^\X If {you filings be sprinkled over a 
bar-magnet, these will be found to adhere about the ends, 
themselves in a certain order, as seen by Fig. 
120. These extremities, where 
the magnetic action seems con- 
centrated, are termed the poles 
of the magnet. 

Let a magnetized bar or needle 
of steel or iron be suspended by 
a thread, or balanced on a pivot. 
Fig. 121, so that it shall be 
free to revolve in a horizontal 
direction, and, it will be found 
after a few oscillations to arrange 
itself in a north and south 
direction. If, now, this be 



arraiigmg 




Were the properties of the Magnet known to the ancients ? Origin of the 
term Magnet? What is said of the power of lodestone in reference to the form- 
ation of Artificial Magnets ? What does Fig. 120 show ? What are the poles 
of the magnet? How will a magnetized bar, free to move, arrange itself? 



THE MAGNET. 155 

turned by the hand, so as to reverse the ends, these will be 
found at once to return to their former position when the force 
is removed/ From the direction which these poles always take, 
the one pointing to the north is commonly termed the north 
pole, and the one to the south, the south pole, and are marked 
with the corresponding initials, as shown in Fig. 121. 

126. The like poles of two magnets repel^ and the unlike 
attract each o/Aer. — Thus, in Fig. 122, if to its north pole 



Fig. 122. 



^v.^S* 




the south pole of a second magnet be presented, the two poles 
will strongly attract each other ; but if the north pole of the 
magnet. A, be presented to the same pole of the other rotating 
magnet, the latter will be repelled, as shown by the dotted 
space ; and, if both magnets be free to move on a pivot, the 
repulsion will be shown to be mutual. Thus two magnetic 
fluids are supposed to exist in the molecules of the metal, 
kno^vn as the austral and boreal fluids, and corresponding 
in their effects to the positive and negative electricities of the 
succeeding section. 

127. A magnetized steel bar induces Tnagnetism in 
another iro?i or steel bar in contact with it. — If a straight 
magnet. A, Fig. 123, be applied to a bar of iron, B, this 
will also assume magnetic properties, and become itself a magnet, 
as may be shown by applying to its lower extremity another bar 
of iron, which will be attracted, and also rendered magnetic. 

State the proposition. Illustrate this attraction and repulsion of the magnet 
by Fig. 122. What fluids are supposed to exist in the magnet ? Give propo- 
sition, § 127. 



156 



THE MAGNET. 



In this case no transfer of the magnetic fluid is made, but simply 
an influence is exerted by the magnet, A, over the fluids existing 
in a combined and latent state in the metals, B and C, whereby 
these fluids are supposed to be separated in each molecule of 
these metals, and rendered free. Thus, each one of 
these invisible particles of the iron becomes itself a 
magnet with north and south polarity. The arrange- 
ment of these elementary magnets may be shown 
by the light and dark shading of the bar, B : the 
former indicating the north, and the latter the south 
pole of each atom. Thus, the like poles of these 
several little magnets are all seen pointing in the 
same direction, causing the extremities of the 
magnet to be always of ojoposite polarities. This 
theory explains the cause of the increase of the 
magnetic force towards the ends of a magnet, and 
the entire want of this in the centre ; for the 
last row of atoms, having no others to oppose their 
action, will exert the ordinary attractive and repulsive effects of 
free magnetism, while, in the centre, this is entirely destroyed 
by counter forces of the adjacent atoms. 

The flict of the magnetic fluids being inseparable from the 

atoms of a magnetic bar, 

^_ ^^/^- ^^ j^^* supposed, ex- 

/f ^^^^ ^^^^^^ 2^.~. ,„, „ ,„ Z,i plains also the fact, that 

wdien such a bar is bro- 
ken in tw^o parts, as seen in Fig. 124, each of these parts becomes 
a perfect magnet, having opposite polarities. Thus, as shown 
in the figure, the fractured end of each piece at once exhibits 
a polarity the reverse of that at the extremities of the original 
magnet, although at this middle point, where s and n join, no 



Illustrate this by Fig. 123. How is the effect of the magnetic influence in 
this case supposed to be produced ? State the theory of the magnet, as 
illustrated by Fig. 123. What does the fact of these fluids being inseparable 
from the particles of the magnet explain ? 



THE MAGNET. 



157 



magnetism could be detected before breaking. If these pieces 
be again divided, and then subdivided, the same results of 
perfect magnets will be seen. 

128. Soft iron acquires and loses magnetism far more rea- 
dily than hardened steel. — Thus, in Fig. 123, the bar of soft 
iron, B, retains its magnetic properties only so long as it is in 
contact with or proximity to the magnet. Remove it from this, 
and the separated magnetic fluids at once come together, and are 
thereby rendered neutral as before. This is shown by C falling 
off as soon as B is separated from the magnet A. If a piece 
of hardened steel be brought in contact with the magnet, the 
inductive effects of this will operate more slowly, requiring some 
time before the remote end will have acquired its opposite 
polarity. Hence it is that electro-magnets, where the magnetic 
effects imparted require to be instantaneous, are 
made of soft iron, while common lifting magnets 
are prepared from hardened steel. 

129. Artificial Mofffiets Sire made 
from steel, and are of a variety of 
forms, depending on the uses to which 
they are to be applied. The U- 
magnet, Fig. 125, is a more com- 
mon form for a lifting magnet, since 
the opposite poles or extremities are 
brought near together and may be 
joined so as to exert their united 
forces on the body to be raised. 
The iron bar which unites the two poles, is called the armature. 
A series of such magnets firmly joined together, as shown 
by Fig. 126, constitutes a Magnetic Battery. 

To Magnetize a Steel Bar. — If straight, place the middle 



Fig. 126. 



Fig. 125. 





What is said in regard to soft iron and hardened steel ? Why are electro- 
magnets made of soft iron ? Of what are Artificial Magnets made, and why 
usivilly made in the form seen in Fig. 125 ? What is the armature ? What is a 
Magnetic Battery ? 

14 



158 MAGNETISM OF THE EARTH. 

of the bar on one of the poles of either a straight or a U- 
magnet, and draw one end of it over the pole a number of 
times ; the direction of the motion being always from the middle 
to the end. Then turn the bar in the hand, and pass the 
other half over the other pole of the magnet in the same way. 
If the bar is thick, the process may be repeated with its 
different sides. The end which has been drawn over the 
south pole of the magnet will now possess north polarity, and 
the other extremity south polarity. A U-magnet is more 
readily charged by drawing over it the poles of a steel U- 
magnet of corresponding w^idth, from the bend to the extremi- 
ties. This should be repeated several times, recollecting 
always to draw the bar in the same direction. When it is of 
considerable thickness, turn it, and repeat the process with 
its opposite surface, keeping each half applied to the same pole 
as before. To withdraw the magnetism from a steel bar it is 
only necessary to reverse the above processes.* 

In order to retain and increase the magnetism of a magnet, 
its tAYO poles should be joined by an armature, through which 
these may react on each other ; otherwise, the two fluids in the 
steel bar will gradually unite and become neutral. 

130. Magnetism induced by the Earth. — If a straight bar of 
soft iron, about two feet in length, be placed at the angle of the 
dip of the magnetic needle (§ 133), it will gradually acquire 
magnetism from the earth : the lower extremity becoming a 
south, and the upper a north pole, as may be shown by bringing 
near the ends one of the poles of a needle. In this way tongs 
and various articles of house furniture, the tools of a work- 
shop, etc., often become magnets. In such a position, induc- 
tion of magnetism by the earth is greatly facilitated by blows 

* Davis' Manual. 

State the process by which magnetism may be imparted to steel bars. 
How may a bar of soft iron be rendered magnetic from the eai'th ? 



MAGNETIC EQUATOR. 159 

with a hammer, twisting, etc., so as to cause a vibration or 
movement among the particles of the metal. This process of 
magnetizing is most effective when the bar rests upon a mass of 
iron. The permanency of a magnet depends no doubt on the 
hardness of the metal and the compactness of the particles, 
which thereby prevents the ready union of the two fluids in the 
particles of the metal. 

Blows upon a magnet, when placed in a horizontal direction 
east and west, will often remove its magnetism. So blows from 
the fall of a magnet may serve to destroy or injure it. Heat, 
also, weakens, while cold serves to increase, the strength of a 
magnet. 

131. Direction of Magnets. — As we have already 

shown, a magnetized bar, when balanced 

^'^" ^'^^' on a pivot and free to move, as shown by 

? ' ~N Eig. 127, takes a nearly north and south 

I direction, obedient to a force termed the 

1^^^^^ earth's magnetic force. Such a magnet of 

the proper form constitutes the magnetic 

needle. This force, which gives definite direction to the magnet 

in every quarter of the globe, is found to reside near the poles 

of the earth, and to be a magnetic force. Thus, the earth is 

supposed to be a huge magnet, with its opposite poles coinciding 

nearly with those of the earth itself; that at the north being 

termed the "tnagnetic north pole^ and the other at the south 

the magnetic south pole. This supposition is confirmed by the 

fact of the magnetic intensity being greatest at a point near the 

pole of the earth, and gradually diminishing, as the equator is 

approached, until a neutral point is reached, similar to that 

between the poles of an artificial magnet. 

This neutral line constitutes the magnetic equator, and 

What are some of the ways in which the magnetism of a bar may be in- 
jured or destroyed ? What is the magnetic needle ? Where is this magnetic 
force found to reside ? What is said of the supposition in regard to the earth 
as a magnet ? 



160 



MAGNETIC NEEDLE. 



fonns an irregular circle around the earth, deviating more or 
less from the terrestrial equator, crossing it at certain points, 
and passing sometimes north and sometimes south of this. 
Thus, in the Atlantic Ocean, at 25° 40' west from Greenwich, 

the magnetic equator reaches 
a point of 14° south lati- 
tude, ai)proaching the ter- 
restrial equator, as it extends 
west, until it meets this at a 
point 117° 40' w^est from 
Greenwich, when it again 
makes a southern curve. At 
172° 20' west longitude, it 
crosses the earth's equator, 
passing along in north lati- 
tude until it reaches 20° 20' 
east longitude, where it again 
intersects the terrestrial equator, and so traversing in an ir- 
regular curve around the earth. The course of the magnetic 
equator, as well as the position of the magnetic poles, may be 
learned from Fig. 128. 

132. The Declination of the Magnetic Needle. — Since the 
magnetic axis does not coincide w^ith that of the earth, the mag- 
netic needle is also found to vary at most points from a true 
north and south line. This variation of the needle, at any 
place, is termed its declination. This declination is east or 
mest, according as the magnetic needle deviates towards 
one side or the other of tlie true astronomical meridian. ^^ 
That point on the earth's surface where the two meridians 

* Thus, in Fig. 128, S N may represent the meridian of a place, and S K 
the magnetic meridian ; then S P S will be the declination at any point. 




Wliat is the magnetic equator and its course? Point out this, as shown by 
Fig. 128. What is meant by the declination of the magnetic needle ? Illus- 
trate tills by Fig. 128. What is the line of no variation ? 



DECLINATION OF THE MAGNETIC NEEDLE. 161 

coincide, is termed the line of no variation^ as here tlie needle 
points in a true north and south direction. This line appears 
to traverse the surface of the globe, passhig in 1660 through 
the meridian of London, and from that time moving westward 
until 1818, when the magnetic needle at that point reached 
its maximum decimation, 24° 18', and has since been return- 
ing eastward to its former direction. 

Such declinations of the magnetic needle, occupying a long 
series of years for their completion, are termed secular declina- 
tions. =^ Besides these, it has also daily variations, of only a 
few seconds of a degree, which are supposed to be caused, in 
some way, by the agency of the sun's heat upon the earth's 
surface. In these the south pole of the needle moves towards 
the west from sunrise until about an hour afternoon, when it 
retrogrades towards the east until eight o'clock in the evening, 
after which it remains nearly stationary until sunrise. The 
Aurora Boreahs is found also to cause the needle to vary con- 
siderably at times ; the extent of these variations, or perturba- 
tions as they are more commonly termed, corresponding with 
the height to which the streams of auroral light ascend above 
the horizon. Electrical discharges also often affect the direction 
of the needle; sometimes reversing its poles or destroying 
entirely its magnetism. 

133. The Inclination or Dip of the Magnetic Needle. — 
From numerous observations made at various points, we are led, 
as already remarked, to regard the earth as a great magnet, whose 
poles are situated within a few degrees of the geographical poles, 
and whose equator, or line, where the magnet is equally attracted 
and maintams a horizontal position, as forming an irregular 

* The declination of the needle at Boston at the present time, January, 
1856, is 10=^ 54:' west of north. 

Does this change from time to time ? What are secular declinations ? How 
are the daily declinations of the magnetic needle supposed to be produced ? 
"What is said of the Aurora Borealis in reference to the magnet ? Of lightning ? 
What is the dip of the magnetic needle ? 

14* 



162 



DIP OF THE NEEDLE. 



circle about the earth in the vicinity of the geographical equa- 
tor. The magnetic poles of the earth act upon those of the 
magnetic needle in a manner similar to those of a powerful 
artificial magnet, causing the needle to be attracted and incline 
toward either pole, w^hen made to approach it from the equator. 
This inclination of the magnetic needle is termed its dip. Fig. 
129 represents a dipping needle for showing this dip of the 
magnet. This is so balanced as to move in a 
horizontal or vertical direction. The dip va- 
ries w^th the latitude approaching the mag- 
netic pole. Thus, at Boston, situated in 
about 42° 20' north latitude, the dip is at 
present 74° 21', and increases as we advance 
north, until, in the vicinity of Hudson's Bay, 
in about latitude 70° 5', it was found, by Sir 
James Boss, in 1831, to be 90°, or vertical 
wath the horizon. The varying positions of 
the needle, with reference to the magnetic 
poles of the earth, are shown by Fig. 128. 

134. Instruments for measuring the declina- 
tion are sometimes termed Declination Com- 
passes. These are of various forms ; as the com- 
mon surveyor's compass, the mariner's com- 
pass, etc. The Surveyor^ s Compass is usually 
a circular box, attached to a universal joint, 
placed upon a stand of three legs. Upon the bottom of this 
box, beneath a nicely-balanced magnetic needle, is pasted a 
circular card, graduated and marked to denote the points of 
compass, like that seen in Fig. 198, Electro-Magnetism. This 
card is so arranged that its north and south points shall be 
parallel to, or in a line with, a sight or telescope. When the 
sight, or telescope, is in a range with the object, the angle 
which the needle makes with the north and south points of the 




How does this vary in different latitudes ? Describe the Surveyor's Com- 



THE COMPASS. 163 

card is the angle of declination, or the bearing of the object 
from the magnetic meridian. The Mariner^ s Compass is 
another form of the declination compass, used for directing the 
course of a ship at sea. The box for the needle is so arranged 
as to keep constantly horizontal, and secure this as far as pos- 
sible from the attractive influence of the iron of the vessel. 

Iron was formerly supposed to be the only substance subject 
to magnetic influences. The researches of Dr. Faraday have, 
however, disclosed a very wide circle of bodies subject to the 
same influence. It is indeed difficult to guess at the limit which 
may exist to the power of magnetism in controllmg or influ- 
encing molecular forces. The elaborate investigations of this 
distinguished philosopher have opened out a rich field of prom- 
ise. A force which a few years ago was supposed to influence 
pieces of iron only, is by these researches shown to act upon 
almost every form of ponderable matter.^ This will be illus- 
trated in a subsequent section. 

*Bird. 



Describe the Mariner's Compass and its proper position. What was formerly 
supposed in regard to iron ? What have the late researches of Dr Faraday 

dis 



ELECTRICITY. 165 



INTRODUCTION TO ELECTRICITY, AND DESCRIP- 
TION OF INSTRUMENTS.* 

135. Natural phenomena usually excite interest in propor- 
tion to the sublimity and mystery which attend their exhibition. 
This is especially true of electrical phenomena. No agent 
in nature presents itself under a greater variety of wonderful 
forms than Electricity : and yet with the essence and proper- 
ties of none are we less acquainted. Now its secret agency 
works silently in the production of the vapor which rises to 
form the storm-cloud, and now again is seen in the terrific 
lightning which darts from this same cloud. In one form its 
mysterious power is exerted in effecting the decomposition of 
organized matter, and the separation of this into its original 
elements ; in another, in the recomposition of these same ele- 
ments to form new compounds. Here we behold this wonderful 
agent obedient to the will of man, — a vehicle of thought to 
bear with the speed of thought tidings to distant regions ; 
while, in another quarter, this same agent is seen traversing 
the heavens and lighting up the polar zone with the brilliant 
coruscations of the Aurora.* 

Thus has the identity of Electricity with the most wonderful 
phenomena of nature ever rendered it of interest in all ages, 
and to all classes of community. It is, however, but little 
more than a century since the attention of philosophers was 
particularly drawn to this subject, and the true foundation of 
Electricity as a science laid. 

136. About the year 1730, Mr. Stephen Grey, of England, 

* This introduction may be omitted, if thought desirable by the instructor. 

What is said of the interest which natural phenomena excite ? What is 
said of the forms under which Electricity appears ? Has Electricity been re- 
garded with interest in all ages ? How recently may the foundation of this as 
a science be said to date ? 



166 THEORIES OF ELECTRICITY. 

commenced the first systematic course of experiments upon Elec- 
tricity with a rude apparatus consisting mainly of a glass tube 
and cloth rubber. The result of numerous experiments led 
Grey to infer that all material bodies belong to one of two 
classes : electrics^ or such as are capable of excitation by fric- 
tion, and non-electrics^ or those incapable of this. Du Fay, a 
sagacious French philosopher, repeated these experiments of 
Grey, and showed that nearly all substances may become elec- 
trics under certain conditions. Soon after, he proposed his 
celebrated theory of two electricities, — one produced by the 
friction of glass, precious stones, etc., which he named vitreous ; 
the other from amber and resins, and called by him resinous 
electricity. 

No discoveries in science were ever hailed with more delight, 
or reflected a brighter glory on the discoverer, than those of 
Dr. Franklin in Electricity. It was about the year 1754 that 
this profound philosopher published the result of a series of 
investigations which alone have served to render him immortal, 
and make his name the pride of every American. 

Franklin denied the truth of the theory advanced by Du 
Fay, and maintained the existence of a single fluid which tends 
to distribute itself equally over all substances. Bodies con- 
taining more than their natural share are said to be positively 
charged, or to contain positive electricity ; those less, nega- 
tively charged, or to contain negative electricity ; the former 
corresponding to the vitreous, and the latter to the resinous 
electricity of Du Fay. Thus bodies in the former state are 
disposed to part with their excess of fluid and share it with 
those about them which are less highly charged ; while those 



What is said of Grey and his experiments ? "What discovery did his experi- 
ment lead to ? What theoi-y did Du Fay propose ? What is said of the discov- 
eries of Franklin in Electricity ? What theory did Franklin maintain ? When, 
by Franklin's theory, are bodies said to be positively charged ? When 
negatively charged ? What tendency have electricities in bodies dififerently 
charged ? 



ELECTRIC INSTRUMENTS. 167 

in the latter state tend to receive from surrounding objects 
until the diffusion becomes equal ; hence the effort to restore 
the natural state or equilibrium of the fluid when disturbed, 
gives rise to the phenomena of the electric spark, or lightning's 
discharge. 

This theory of a single fluid, while it commends itself for 
its simplicity, is deemed inadequate to explain many of the 
details of electric phenomena, and the theory of two fluids has 
accordingly been more generally adopted, — these taking the 
names positive and negative, as applied by Franklin to the 
single fluid. 

Among the more practical results of Franklin's investiga- 
tions was the discovery of the identity of lightning and elec- 
tricity, and the utility of pointed metallic rods for protecting 
buildings against the dangers of the same. From the period 
of Franklin's discoveries. Electricity began to assume an impor- 
tant rank among the sciences : and from a fierce and dreaded 
element, has come to serve as a powerful contributor to the 
physical and social delights of man. 

DESCRIPTION OF ELECTRIC INSTRUMENTS. 

137. Within a few years the facilities for illustrating electrical 
science have greatly improved, and the rude machines of 
Franklin and Du Fay have been superseded by a far higher 
and more efficient order of instruments. Some of the more 
important we shall here briefly describe. 

The Plate Electric Machine^ Fig. 131, may be regarded 
as the most convenient and efficient instrument for exciting 
and collecting the electric fluid. A circle of thin plate 
glass, G, nicely cut and polished, is placed on a steel shaft 
between two brass shoulders. One of these shoulders is sol- 

What is said of this theory of a single fluid ? State some of the practical 
results of Franklin's investigations. From the period of Franklin's discov- 
eries what is said of the rank of electricity among the sciences ? What is said 
of the Plate Electric Machine ? 



168 



PLATE MACHINE. 



dered firmly to the shaft, Avhile the other is movable, and screws 

up against the glass plate. Two morocco leather-washers, 

against which these shoulders bind, are stuck firmly on the 

plate. 

^ This shaft and plate are mounted on wooden posts, W W, 

standinor on a beautiful cross basement. Two rubbers, R, of 



Fig. 131. 




buff leather, coated w^ith amalgam, are fastened to brass plates, 
w^hich are pressed against the glass plate on either side by 
brass springs, S. These springs are set in a hub placed on the 
top of the negative post, and held firmly by the negative ball, 
N, which screws down against it. The prime conductor, P, 
rests on a second insulated glass post ; from the end of this, 
next the plate, extend, in the larger machines, two rows 
of points, for taking the electricity from the glass surface. 
From the other end a ball and sliding-rod, U, extend, which 
in the large machines are movable, and may be drawn out at 
pleasure. A silk bag, in which the plate revolves, is arranged 
as seen in the cut. 

138. The order of setting up and manner of operating the 
Plate Machine. — Place the glass plate evenly against the 

Describe the Plate Electric Machine. 



PLATE MACHINE. 169 

fixed shoulder of the shaft, and screw the other shoulder 
against it with a moderate force ; place the two wooden or 
shaft posts on the shaft, and lower them together into the holes 
of the basement, and secure firmlj by the nuts and washers 
underneath. Screw the handle, T, to the shaft ; place the rub- 
bers on either side of the glass plate, with the oil-silk flaps down- 
ward, and slip the hub on the screw at the top of the negative 
post, and screw on the negative ball to bind it. Regulate the 
pressure of the springs by the screw-balls at the side, and let 
this pressure be light. Attach the rows of points either before 
or after the prime conductor has been placed on its post, taking 
care that these stand even, so as not to touch the glass. Place 
in the opposite end the sliding-rod and ball, and arrange the 
silk bag to hang free and even when the plate is turned. 

139. Before operating these machines^ see that the shaft- 
posts rest even on the basement, so as to cause the plate to re- 
volve true and without touching the rows of points. These points 
should not be allowed to become bent or blunted. Rub the glass 
posts briskly with an oily silk rag, and also the glass plate, 
taking care to keep this latter free from any streaks of amalgam 
which may adhere to its surface. 

Whenever the amalgam becomes dry and hard upon the rub- 
bers, remove these, and with a knife ruff this up and soften 
with tallow, smoothing it again ; or, if necessary, spread on 
more amalgam. 

If the floor of the room be dry, let the chain leading from 
the negative ball extend a good distance along this, or it may 
connect with a water or gas pipe of the building. Should any 
points or rough edges be found about the prime conductor, or 
the jars used for holding the fluid, these should be burnished 
smooth and covered with a coat of varnish. The escape of the 
electricity in the dark will point out such defects.* 

* These electric machines should be used with care, and neyer allowed as 
mere playthings for boys. A reckless Jehu at the crank may do more injury 
in five minutes than a careful operator in as many years. With a proper re- 

15 



170 



ELECTRIC INSTRUMENTS. 



While in use every part of an electric apparatus should he 
kept free from dust ; and never should this be placed in a 
damp room or exposed to the vapor from unstopped bottles of 
acids and other liquids. 

140. The Leyden Jar^ Fig. 132. This is used for collecting 
and holding a quantity of the electric fluid. To charge this 
positively, let the brass ball connect with the prime conductor 
of the Electric Machine while in action, while the outside coat- 
ing has a free communication with the earth ; a dry and 



Fig. 132. 




Fig. 134. 



Fig. 133. 




varnished table will not aiford this. To discharge the same, 
apply one of the balls of the Plain or Jointed Dischargers^ 
Figs, 133, 134, to the outside coating, and bring the other to 
the knob of the jar.^ 

gard to the above directions, the Plate Machine may be made to operate bril- 
liantly during a damp day and in a cold room ; although a warm and dry 
atmosphere is much more favorable. 

* The theory of the charge and discharge of the Leyden Jar will be given 
in a future section. 



Use of the Leyden Jar ? 
charge? 



How charge this positively? Manner of dis- 



ELECTRIC INSTRUMENTS. 



171 



Fig. 135. 




141. The Electric Battery^ Fig. 135, is a series of Leyden 
Jars, with their inside coatings connected, and also their outside. 

To discharge the same, place one 
ball of the discharger on the brass 
knob at the side of the case, or 
the object through which the dis- 
charge is to be made, and bring 
the other to the ball of one of the 
jars, whereby a communication 
will be formed between the inside 
and outside coatings of the jars, and a discharge effected. =^ 

142. Lane^s Discharger^ Fig. 136, may be used for regu- 
lating the quantity of electricity to be passed 
through a body. This, when attached to the 
prime conductor, or a Leyden Jar, may be 
effected by sliding the rod with the ball so 
as to vary the distance according to the degree 

of the charge to be passed from the jar or conductor. A chain 

may connect the outside ball with the body through which the 

fluid is to be passed. 

The Directing Rod^ Fig. 137, is a convenient instrument 

for directing electricity from the prime conductor to jars, etc., 

arranged upon a table. As it slides in a tube which is 

jointed, it may be 
easily directed at 
pleasure. When in 
connection with a 
jar or battery, care 

should be taken against attempting to adjust it with the hand ; 

as in such a case a severe shock may be received. 





* If any one of these jars be of thin glass, and too highly charged, the fluid 
may force its way through the side, and so puncture and destroy the jar. 



What is the Electric Battery ? Manner of discharge ? Describe the manner 
of using Lane's Discharger. For what is the Directing Rod used ? 



172 



ELECTRIC INSTRUMENTS. 




The Pla7ie and Graduated ^'^''^^'^^ 

Fig. 138. Pith-Ball Electrometers. — 

A Figures 138, 139, are used to 
denote the kind and degree of 
tension of the electric fluid in 
^^ Leyden Jars and other electri- 
fied objects. 
143. By means of the Hydrogen Generator^ Fig. 140, many 
interesting experiments in the explosion of inflammable gases, 
by electricity, may be performed. To prepare this for use, fill 
the jar with a mixture of about one part of sulphuric acid to 
twelve or sixteen of water, so that 
when the bell is forced into this and 
empty, the liquid shall rise nearly to the 
shoulder of the jar. Before lowering 
into this, suspend within the bell the 
copper vessel filled with granulated 
zinc.=^ See that all the atmospheric air 
is expelled from the bell before any 
flame is brought near the jet attached to 
the stop-cock. This may be done by 
opening this and drawing ofi" the gas 
twice. 

To Jill the Electric Cannon or Gas- 
Pistol for firing by the electric 
spark. — Hold these over and just above 
the stop-cock or long jet attached, and 

J..- ———'A let up the gas; then, before inverting, 
, m^^-^ *-*^^^ insert a nicely-fitted cork. The cannon 
or pistol is now filled with a highly 

* A roU. of sheet zinc may be used with greater convenience and economy. 
See Fig. 276. A block of zinc is shown in the cut. 




Use of the Pith-Ball Electrometers ? How is the Hydrogen Generator pre- 
pared for use ? Describe the process of fitting the Gas-Pistol for exploding by 
electricity. 



ELECTRIC INSTRUMENTS. 



173 



explosive mixture of hydrogen gas and atmospheric air. Pure 
hydrogen will not explode, and in these experiments requires 
to be mixed with a due proportion of atmospheric air. Should 
the experiment not succeed, from the too great purity of the 
hydrogen w^ithin the barrel, draw the cork nearly out, and, with 
the pistol inverted, open end down, blow gently from the mouth 
for a moment across the end of the barrel, and again replace the 
cork. This will serve to mix the gases within and render them 
explosive.* 

144. The Gold- Leaf Electrometer^ Fig. 141, is a delicate 
instrument for detecting the kind and degree of electricity in 
bodies. The metallic cup is for showing the 
effect of evaporation on the electric state of 
bodies. The poiiit serves to attract electricity 
from the atmosphere, and so determine its 
positive or negative state ; while the con- 
denser attached to the side detects the 
presence of slight quantities of the electric 
fluid, which through its influence show them- 
selves by the divergence or collapse of the 
gold leaves. This is a serviceable instru- 
ment for investigating the electrical state 
of the atmosphere, and for showing the ef- 
fects of induction at a distance. 



Fig. 141. 




* The platina sponge attached to the stop-cock in the cut is not used in 
these experiments. 

A mixture of two parts of hydrogen and one of oxygen may be used for 
filling these instruments. This forms a compound much more explosive and 
sure of ignition, and may be used from a bag in which it has been previously 
mixed. 



Uses of ihQ Gold-Leaf Electrometer ? 

15* 



174 ELECTRICITY. 



MECHANICAL ELECTRICITY. 

145. Few persons can have failed to notice the singular prop- 
erties which glass, resins, the fur of animals, and a variety of 
bodies, acquire when rubbed with dry silk or woollen cloths. 
If a glass tube, for instance, be rubbed briskly with a silk 
handkerchief, and then held over small fragments of pith, 
paper, etc., lying upon a table, these will be seen to fly up and 
adhere to its surface for a time, and then fall off; and, after 
a short interval, again fly to the surface of the tube, and so 
continue to pass and repass between the two surfaces. While 
in this state, if the tube be held near the face, a tickling sen- 
sation, like that from the touch of a cobweb, will be experienced. 
If the fur of a cat be stroked with a piece of silk, in the dry 
air of a dark room, flashes of light will be seen, accompanied 
by a slight crackling sound. 

These and a variety of kindred phenomena are attributed to 
the agency of an exceedingly subtle fluid, or mysterious prin- 
ciple, called Electricity^ with which all matter seems endowed. 
Like heat the electric fluid tends to distribute itself equally 
through all material bodies, and it is only when this equilibrium 
is disturbed by friction, heat, and other causes, that its effects 
i)8Come visible. 

146. Electricity by Friction. — Friction is the more common 
method of exciting this fluid ; a,nd by this means all bodies 
may be made to produce it. 

Experiment. — Rub the glass tube with the silk handker- 
chief, as before mentioned ; electricity will soon be formed on 

What singular phenomenon does a glass tube present when rubbed with a 
])iece of silk, and then held near small fragments of pith, paper, etc. ? Effect 
of holding such a tube near the face ? What is stated in regard to the fur of 
a cat ? To what agency are these and a variety of kindred phenomena attrib- 
uted ? How does electricity distribute itself through bodies ? What is the 
more common method of exciting electricity ? State the experiment with the 
glass tube and metallic rod. 



ELECTRICITY. 175 

its surface, which will show itself by the emission of slight 
sparks and the attraction of light substances. Treat a smooth 
metallic rod in the same manner, and electricity will also be 
produced, yet without any visible effects. In the former 
instance the glass, from its non-conducting quality, prevents the 
escape of the fluid formed on its surface ; while, in the latter, 
the conducting property of the metal allows it freely to escape. 
Hence, all such bodies as glass, resins, silks and furs, which 
are capable of retaining the electric fluid, are called non-con- 
ductors ; while those which allow it freely to escape, as the 
metals, water, etc., are termed conductors of electricity. 

Expei^iment a. — Warm a sheet of common writing-paper 
before the fire, and apply friction with a piece of India rubber, 
^^•hen the paper will soon become so electrified as to adhere to 
the walls of the room. 

The same effect may be produced by the friction of various 
cloths. Thus, woollen cloths, when brushed in cold and dry 
weather, often become highly electric, and attract the particles 
of floating dust. 

Experiment b. — Agitate some mercury in a strong glass 
tube ; the friction of this will render the tube electric. 

Experiment c. — Blow with a common bellows against the 
ball of a delicate electrometer ; the friction of the air will 
often excite a sufficient quantity of electricity to become per- 
ceptible. 

The friction of currents of air against each other and the 
clouds is thought to be one source of free electricity in the upper 
regions of the atmosphere. 

147. The friction produced by the escape of steam, under a 
high pressure, from a rough and irregular opening in a steam- 
Why does the glass become excited, while the metal does not? What are 
such bodies as glass, silks, furs, etc., called, and why? Metals, water, etc. ? 
Give Experiment a. What is said of woollen cloths brushed in cold and dry 
weather? Give Experiment b. Give Experiment c. What is said of elec- 
tricity produced by steam ? 



176 TWO ELECTRICITIES. 

boiler, is one of the most efficient sources of electricity 
known. 

Thus, by means of a small insulated, steam-boiler, three and a 
half feet long by one and a half in diameter, Mr. Armstrong, 
of England, was enabled to obtain a spark " fifteen inches in 
length," and to charge Leyden Jars with a rapidity equalling 
that of the most powerful plate electric machines. 

148. Electriciti/ is of two kinds. — We have already, in the 
Introduction, spoken of the two theories of Electricity proposed 
by Du Fay and Eranklin. We adopt the theory of the former 
as affording a more satisfactory explanation of many of the 
details of electric phenomena. 

Experiment. — Suspend a pith ball by a long silk thread, 
and hold near it a glass tube excited as before mentioned. The 
pith ball will immediately fly to the surface of the tube 
'°" ■ and adhere for a moment, and then be repelled from it 
to a considerable distance. Excite now the wax cylin- 
der, Fig. 142, by means of a piece of diy flannel, and 
hold it toAvard the pith ball wdiich has been repelled from 
the glass ; . it will instantly be drawn to the wax, and 
then repelled from it, and again attracted by the glass 
tube, from which it will soon be again driven, and seek 
the wax cylinder ; thus continuing to pass and repass 
with much energy between the two electrified bodies. 

149. Thus we see that the electrical states of the glass 
and the w^ax in this experiment are widely different, — 
each attracting what the other repels. The theory 
of Du Fay in this instance supposes the production of two 
opposite electricities ; that, formed upon the surfiice of the 
glass being called vitreous or positive., and that upon the wax 



Whose theory of electricity is adopted in this work ? Give the experiment 
with the pith ball when acted on by excited cylinders of glass and wax. 
What is said of the electrical states of the glass and wax in this experi- 
ment ? What explanation of these phenomena does the theory of Du Fay 
give ? 



ELECTRIC MACHINE. 177 

resinous or negative electricity. These two electricities have a 
strong mutual attraction : and, when separated bj friction and 
other causes, tend to combine again and render each other neu- 
tral or latent, which is their natural state. 

Thus, in case of the pith ball, when charged with the posi- 
tive electricity of the glass, it was strongly attracted by the 
opposite negative electricity of the wax, when it parted with its 
charge and became negatively electrified ; whereupon it was at 
once attracted by the opposite electricity of the glass, thus serv- 
ing as the vehicle for conveying to and fro the two fluids, and 
eifecting a neutrality or equilibrium. 

These tuo electricities are alioays ^produced simid- 
taneously^ the one in the rubber, and the other in the body 
rubbed. In the example of the glass tube, while it acquired 
positive electricity from the friction of the silk, the latter 
became in an equal degree negatively electrified. So of the 
friction of the wax cylinder ; while this became negatively 
excited by the flannel, the latter acquired a like share of pos- 
itive electricity. 

150. Theory of the Electric Machine. — We have already 
described the construction and mode of operating this instrument, 
and it remains only to explain briefly the theory of its opera- 
lion. This acts on the principle of the glass tube, difiering 
only in the greater facilities it aflbrds for creating friction and 
collecting the electric fluid. As the plate is revolved, friction 
decomposes the electricities of the rubber, its positive adher- 
ing to the plate, while the negative remains behind in the rub- 
ber. When the surface of the glass with its charge of positive 
electricity comes opposite the row of points, the negative 
electricity is draicii from the conductor^ and combines with the 
positive fluid upon the glass, thus leaving the prime conductor 



How are these electricities always produced ? What is said of the electrical 
states of the rubber and the body rubbed ? Explain the theory of the action 
of the Electric Machine. 



178 ELECTRIC REPULSION. 

charged with positive electricity. If the rubber be insulated, 
the quantity of electricity is soon exhausted ; hence the neces- 
sity of the chain for allowing a constant supply to pass up to 
it from the earth. 

To charge a jar positively^ we have only to connect the 
inside coating with the prime conductor, while the outside coat- 
ing has a free communication with the earth, and work the 
machine. To charge ajar negatively^ XQmo\Q the chain from 
the ru1)ber, and attach it to the prime conductor, and then bring 
the knob of the jar to the negative ball, and proceed as before. 
151. Bodies charged with like electricities repel^ and ivith 
unlike^ attract each other. — Experiment. — Place the Pith- 
Ball Electrometer., Fig. 138, on the prime conductor, and work 
the electric machine. The balls will immediately become 
charged with the same (positive) electricity, and separate as 
far as possible, as seen in Fig. 131. If, now, the hand or any 
negative body be held towards them, they will be strongly 
attracted, and follow it as though possessed of intelligence. 
Arrange the machine for obtaining negative electricity by 
removing the chain from the negative to the 
Fig. 143. prime conductor, and place the balls on the 

, , ,\;(vl||( li/ , negative conductor, and proceed as be- 

fore. The balls will now separate, from 
being again charged with the same (neg- 
ative) electricity. 

Experiment a. — The same property of 
repulsion in matter similarly electrified, may 
be illustrated by the ridiculous figure of the 
Long-haired Man., Fig. 143. Arrange this 
as in the last experiment. When electri- 
fied the hair stands on end, and each fibre, 



Use of the chain attached to the rubber ? How may a Leyden Jar be charged 
positively ? How negatively ? State the proposition, section 151. Give the 
experiment with the Pith-Ball Electrometer. Give the experiment with the 
Long-haired Man. 



ELECTRIC llEPULSION. 



179 



as if in a state of repulsion from its neighbor, maintains an 
isolated and erect position. 

Experiment h. — If some Lirds cut from light pith, and 
fastened by fine thread, be placed upon the ball of an Electrom- 
eter Jar, Fig. 144, and the bent wire be so adjusted as to bring 
its ball within an inch of the outside coating, upon charging 
the jar, by connecting with the positive or negative conductors, 




the birds will gradually rise, attaining a higher elevation as the 
tension of the electricity increases, until a discharge, accom- 
panied by a loud report, is eifected between the ball and outside 
coating, when the birds will suddenly fall. 

To render this experiment the more amusing, a fancy sports- 
man may be arranged, as in the 
cut, w^hen at each discharge the 
birds will have the appearance 
of being shot down. 

Experiment c. — Let a boy 
with fine and dry hair stand 
upon the Insulating Stool^ Fig. 
145, and place his hand on the 
excited prime conductor. He 

Give the experiment with the Electrified Birds. How may a person become 
highly charged with electricity ? "What amusing phenomena may a person 
thus charged be made to present ? 




180 DANCING IMAGES. 

•will soon become electrified, causing his hair to stand erect ^ as 
in Experiment a. Sparks may now be taken from the body by 
any person uninsulated, jars charged, and a variety of amusing 
experiments performed, as from the prime conductor of the 
Electric Machine, of which he has indeed become a part. To 
succeed with this experiment all knives and pointed instru- 
ments should be removed from about the person insulated. 

152. Experiment. — A most amusing illustration of electric 
attraction and repulsion is shown by the arrangement seen in 
Eig. 146. Let either of the sets of metallic plates be arranged 

Fig. 146. 




as seen in the cut, the upper plate connecting with the prime 
conductor and the lower with the earth. Place between these 
some fancy pith images, and work the machine so as to regu- 
late the quantity of electricity required ; the images will at 
once commence a lively dance, passing rapidly between the two 
plates, and conveying back and forth the opposite electricities to 
restore the equilibrium.* 

153. Experiment. — Let tw^o jars, of equal size, be charged 
with diflferent electricities, and placed side by side. Suspend 

* These plates should hang even, and any rough edges or particles of fibre 
should be thoroughly removed. A small pin, with the head projecting slightly 
from the foot, will often give a proper balance and motion to these images when 
*' top-heavy." Three forms are shown in Fig. 146. 

Give the experiment "with the Dancing Images. What is the experiuT^ut 
with the Electric Spider ? 



ELECTRIC SWING AND BELLS. 



181 



Fig. 147. 




midway between tlieir knobs, by a fine silk thread, a piece of 
cork or pith, cut in the form of a spider, with threads drawn 
through it to resemble legs, and the whole colored black. This 
will vibrate between the two jars, forming an amusing experi- 
ment, known as the 
Electric Spider ; and, 
in a short time, the 
electricities of the two 
jars will be brought 
together and rendered 
neutral. 

Experiment a. — 
Let a small fancy toy^ 
Fig. 147, made from 
pith, be so suspended as 
to swing between the two brass balls, P and N, just grazing 
upon these.- The ball P is placed on an insulated post, and is 
connected by a chain with the prime conductor, while N com- 
municates with the earth. Work the machine, and the toy will 
be drawn to P, become positively charged, and then repelled to 
N, where it will discharge, and then be again attracted ; and so 
pass and repass between the two balls, con- 
stituting the Electric Swing. 

Experiment b. — Suspend from the prime 
conductor the Electric Bells ^ as seen in Fig. 
148 ; the two outer have a metallic commu- 
nication with the conductor, while the two 
balls or hammers are insulated from it, as is 
also the middle bell, which communicates 
with the earth by a chain. Excite the prime 
conductor and outer bells positively : the balls will be drawn 
to these, and, acquiring a charge of positive electricity, will be 



Fig. US. 



^ 



The Electric Swing, Fig. 147 ? How are the Electric Bells arranged ? Give the 
experiment with these, and the cause of the attraction and repulsion of the 
balls. 

16 



182 



BALANCE ELECTllOMETER. 



Pig. 149. 



repelled to, and attracted by, the middle or negative bell, where 
thej will discharge and be again attracted to the outer bells, 
and again repelled ; thus passing to and fro, and ringing out 
most forcibly the theory of electric attraction and repulsion. 
154. Experiment. — Place the toy known as the Electric 
Swan in a glass basin of water, and connect 
with the prime conductor as in Fig. 149. 
Work the machine so as to render the swan 
and water positively electric. If, now, the 
finger or any negative body be held towards 
its beak, electric attraction will cause it to 
swim after and follow the finger, as though 
possessed of instinct. 

Experiment a. — The repulsion of like 
and attraction of opposite electricities is well 
illustrated by the Balance Electrometer^ Fig. 150. Balance 
the rod and balls by means of the movable ring placed on one 




Fig. 150. 




of the arms. Connect the ball of the insulated post with the 
Leyden Jar, and charge the same from the prime conductor. 
The ball of the balance will now be attracted by the positive 
electricity of that connected with the jar, and, becoming posi- 



What is the experiment with the" Electric Swan ? Explain the arrangement 
of the Balance Electrometer for showing electric attraction and repulsion. 
Cause of the vibrations of the rod and balls when connected with the charged 
jar? 



ELECTRIC ATTRACTION AND REPULSION. 



183 



tively electrified, the balance will be repelled from this and at- 
tracted at the opposite end, where it will deliver its charge, 
become negative, and be again attracted, and again repelled, 
thus continuing to vibrate and carry off electricity until the 
jar is discharged. This experiment may be rendered more 
amusing by fastening a fancy toy on each arm of the beam, and 
so illustrating the game of see-smo. 

Experiment b. — Balance on a point upon the insulating 
stand of the Universal Discharger, Fig. 1^1^ 2i small bell-glass. 



Fig. 151. 




Remove the pincers from the conducting-rods, and in their 
place attach two points ; connect one of the rods with the prime 
conductor, and the other with the earth ; bring the points near 
the sides of the bell, deviating a little upon opposite sides of 
the diameter of this, and work the machine. The electricity 
from each point will charge the surface of the glass near it with 
its own electricity; consequently there will be a repulsion 
between that of the point and glass, and an attraction between 
that of the glass and the point upon the opposite side. Acted 
on by these forces, and being free to move, the bell-glass 
will commence a rapid revolution, discharging its sides upon 



Explain the theory of the revolution of the bell-glass, as shown in Fig. 151. 



184 ELECTRIC INDUCTION. 

the opposite points in its revolutions, and tlius affording a novel 
exhibition of electric attractions and repulsions. 

ELECTRIC INDUCTION. 

155. A remarkable property of the electric fluid is its power 
of affecting the electric state of bodies by its mere approach, 
Avithout any actual communication of itself to those bodies. 

Thus, if a glass tube be excited by friction, and held towards 
some insulated body, to which is attached a delicate electrom- 
eter, free electricity will at once become visible in the body, 
although the tube be held at so great a distance as to preclude 
the possibility of imparting any of its free electricity. 

Experiment. — Let three brass cylinders be arranged on 
glass supports, and provided with pith-ball electrometers placed 
on wires near their extremities, as seen in Fig. 152. Let b be 



Fig. 152. 



^^ 



r^T\ 



placed in the vicinity of the prime conductor. A, while d con- 
nects by a chain with the earth. If, now, A be positively 
excited, it will act to decompose the electricities of b, causing 
its negative to be attracted to the end next to A, while its 
])ositive electricity is repelled to the further extremity ; thus, the 
two sets of pith balls attached to the two ends of b will diverge 
from opposite electricities. The positive electricity of b will, 
moreover, decompose in like manner the electricity of c, and c 
that of d. causing the balls of each to separate as in the case 
of b. The positive electricity of d, being driven off to the 

What remarkable property of the electric fluid is stated in section 155 ? 
Experiment with the glass tube and pith balls? Give the experiment Avith 
tlic brass cylinders and sets of pith balls. Explain the theory of these phe- 



ELECTROPHOROUS. 



185 



earth, will cause the balls upon its further or positive extremity 
to diverge less than those at the other extremity. In this ex- 
periment no electricity passes from A. but the result is caused 
by mere induction, as may be proved from the fact that the balls 
again come together as soon as the excited body is removed. 

156. A?i electrified body tends always to induce an oppo- 
site electrical state in bodies brought near it. — Thus, in the 
last experiment, if the positively excited glass tube be brought 
near one end of the cylinder, the natural electricities of this 
will be decomposed: its negative being attracted to the end 
next the tube, while its positive will be repelled to that most 
remote, causing the balls at each end to diverge from different 
electricities. If, while in this statje, the further end of the 
cylinder be touched by the hand, the positive electricity will 
escape to the earth, and, upon withdi-awing the excited glass tube, 
the cylinder will be found charged with only negative electricity. 

157. Upon this principle of electric induction, may be ex- 
plained the attraction and repulsion shown in sections 152-3. 

The Electrophorous^ Fig. 153. is an instrument whose action 
is due to this same cause. A 
circular metallic basin, B, is 
filled with melted wax, and 
cooled, so as to leave a smooth 
and even surface. This basin 
or wax disc is then screwed 
to the top of an insulated 
glass post. Accompanying 
this is a circular metallic 
plate. A, somewhat less in 
diameter than the disc, and 
provided with an insulated 
handle of glass. 




State the proposition section 156. How illustrated by the previous experi- 
ment? Describe the Electrophorous. 

16^ 



186 INDUCTION. 

Exper'iment. — Rub the wax disc of the Electrophorous 
briskly with a warm iSiannel, or. which is better, whip it with a 
catskin, and the wax will become negatively electrified. Place 
now the metallic plate on this, taking it by the glass handle ; 
remove it, and no electrical change is induced ; place it again on 
the wax, and at the same time touch it with the finger, so as to 
form a communication with the earth ; the electricity of the 
wax will decompose those of the plate, attracting its positive, 
and repelling its negative, which will escape through the hand 
into the earth. Now, raise this again by the handle, and hold 
near the knuckle, when a brilliant spark of positive electricity 
will be received. Place the plate again on the wa>x disc as 
before, and remove ; another spark may be received ; and so the 
experiment may be repeated any number of times, since the 
wax loses none of its negative power, but acts only to induce a 
change in the electric condition of the metallic plate. 

Experunent a. — With the wax disc excited, as in the last 
experiment, write on it with the knob of a jar, charged with 
positive electricity, and then blow upon it a mixture of pow- 
dered red lead and sulphur, from a small bag, (Fig. 153), or 
bellows. The lead powder will adhere to the negatively excited 
portions of the wax, and the sulphur to the positive, or those 
portions touched by the knob, clearly defining the course of this 
by beautifully radiating lines. 

The Electrophorous will often retain its action when once 
charged, for weeks, and is a convenient means of obtaining 
small quantities of electricity for experiment. 

158. Theory of the Ley den Jar. — The Leyden Jar is 
simply a common specie-jar, coated inside and out to the same 
level, with tin-foil, and provided with a wooden cap, and a ball 
and chain connecting with the inside coating. Its uses and mode 
of charge and discharge have been already referred to, § 140 ; it 



Give the manner of exciting th:b, and explain tlie theon' of its excitation. 
State experiment a. Describe the Leyden Jar. 



THEORY OF LEYDEN JAK. 



18T 



Fig. 154. 



now only remains to explain the theory of its operation. This 
is due to the principle of inductioji just explained. 

Experiment. — Place a Leyden Jar upon an insulated 
stand, with a pith-ball electrometer con- 
necting with its inside and outside coat- 
ings, as shown in Fig. 154. Connect the 
inside coating with the prime conductor, 
and work the machine ; the jar will 
become but feebly charged, as indicated by 
the two electrometers. If, now, the 
knuckle be brought near the outside coat- 
ing, sparks will be received from this, 
and the jar become rapidly charged, the 
inside with positive, and the outside with 
negative electricity, and both sets of 
balls will diverge from opposite fluids. 

In this example the positive elec- 
tricity, as it enters the inside, acts by 
induction^ through the non-conducting glass, to rejjel the same 
from the outside of the jar, and attract its opposite negative 
electricity ; but, while insulated, the escape of the positive fluid 
from the outside is prevented, w^hich precludes the entry of the 
same into the inside. Hence the reason of the necessity of a 
free communication betw^een the outside coating and the earth, 
during the act of charging a Leyden Jar or battery. 

159. Glass, resins, air, and all kindred insulating media which 
resist the passage of the electric fluid, but through which this 
may exert its inductive power, are called dielectrics. Through 
these, electricity is supposed to act by a polarization of their 
particles; each molecule of the glass, etc., being thrown into 
opposite electrical states, as in the case of the cylinders, Fig. 




State the expeinment with the Leyden Jar placed upon an insulated stand. 
Wliat will be necessary to enable the jar thus placed to become fully charged ? 
Give the explanation of this phenomena. What are dielectrics? How is elec- 
tricity supposed to act through such bodies ? 



188 



INDUCTION. 



152, and thus, bj a series of attractions and repulsions, an influ- 
ence traverses the intervening medium, which produces its de- 
composing effects bejond. 

160. The force of Induction diminishes rapidly as we 
recede from the exciting cause ; hence, the thinner the glass 
of the Leyden Jar, and the nearer the two fluids are brought, the 
more intense will become the electric charge. If, however, the 
glass of the Leyden Jar, for instance, be too thin, the tension of 
the fluids may be so great as to overcome this resistance, and 
so unite by forcing a passage through the glass, thus punctur- 
ing and destroying the jar. 

Experiment. — Arrange two Leyden Jars as in Fig. 155; 
connect the ball of the upper with the prime 
conductor, and charge the inside positively; 
this will repel the same fluid from its outside 
into the jar below, when both will be found to 
be positively electrified. By arranging a series 
of jars upon an insulated surface, and con- 
necting the outside coating of the one in com- 
munication with the .prime conductor with the 
inside of the next, and so on, the whole may be 
charged, each from the electricity driven off 
from the outside coating of its preceding neigh- 
bor ; the charges, however, becoming more fee- 
ble according to the distance from the prime 
source of induction. 

Electricity resides mainly on the surface 
of the glass in the Leyden Jar, while the 
metallic coatings serve only as conductors of 
it. This may be satisfactorily shown by a jar 
with m,ovahle coatings^ as seen in Fig. 156. 



I 



What is said of the force of induction as we recede from the exciting cause ? 
What is the experiment with \\\o jars seen in Fig. 155? What is said of the 
manner of charging a series of jars from each other ? In the Leyden Jar 
where does the electricity mainly reside ? 



MOVABLE-COATIXG JAR. 



189 



Fig. 156. 




Experiment a. — Apply the ball to the prime conduct- 
or, and charge the jar; lift out the in- 
side coating by the glass tube, and hold it 
suspended, avoiding the metallic ball ; free 
the outside vessel or coating from the glass, 
by inverting on the table ; place the smaller 
within the larger coating, and apply the dis- 
charger. No electricity will be found on 
these metallic surfaces. Now, slip the out- 
side coating over the glass again, taking care 
not to touch with the hand the surface of the 
glass, and replace the inside one as before. 
If the discharger be now applied, a vivid flash 
will pass between the two coatings, showing 
that the electric fluid remained on the su?^- 
face of the fflass, while the metallic coatings served merely 
as conductors for the same. 

161. The tension of electricity varies inversely as the sur- 
face over which it is distributed. 

Experiment. — Let two tin cylinders, say three by eight 
inches, fit one within the other, the inner being provided with 
an insulated handle, and attach a delicate pith-ball electrometer to 
the outer one ; place the whole on an insulated stand, and charge 
with electricity. The pith balls will separate and show the 
degree of tension of the electricity. Now raise the inside cyl- 
inder slowly by the insulating handle, the balls will gradually 
fall, showing a diminution of the electric tension, as the fluid 
becomes distributed over a greater surface ; depress the cylinder, 
and the balls will again rise. After a jar or battery has been 
charged for some time, and then discharged, it will often spon- 
taneously recover its charge to a considerable extent, giving 
rise to what is known as the residual charge.^ This, accord- 
* Wlienever a discharge is made through a body, from one conductor to 

How may this be proved by means of the movable-coating jar, Fig. 156 ? 
How does the tension of electricity vary ? How may this be proved ? 



190 ELECTRIC ILLUMINATION. 

ing to Faraclaj, is caused by the electric fluid which is forced 
into the glass among its particles bj the tension of the electricity 
on its surface, and which returns slowly to the surface as the 
external force is removed by a discharge. From this cause an 
ordinary battery will sometimes accumulate sufficient electricity, 
after a discharge, to give a smart shock. 

LUMINOSITY OF THE ELECTRIC SPARK. 

162. Whenever any considerable quantity of electricity is 
passed through a ?nediimi which resists its passage, light 
and heat are produced. — These are caused by the sudden 
compression of the air or other medium before the electric fluid 
in its rapid passage, whereby latent heat becomes sensible, ac- 
companied by light. The color and course of the electric 
spark vary with the media through which, and the conduct- 
ors between which, it is passed. The following experiments 
require a dark room. 

Experiment. — Unscrew one of the balls from the discharger, 
Fig. 133, and substitute a ball of ivory ; place the other metal- 
lic ball against the outside coating of a charged jar, and bring the 
ivory ball towards the knob of the same ; the light from the 
electric discharge, between the knob and the ivory, will be of a 
deep crimson. Substitute in place of the ivory ball a lump 
of sugar, when the color of the spark will be ichite., and the 
sugar will remain luminous for some time. Fluor spar will give 
a green light. 

another, a chain or wire should lead from the further conductor to the outside 
of the jar. Unless this be observed when the jar is grasped by the hand, the 
body of the operator forms a portion of the electric communication between the 
two coatings, and a severe shock may be received. 



What is meant by the residual charge? How is this, according to Faraday, 
caused ? Under what circumstances are light and heat produced by elec- 
tricity? How are these caused ? How do the color and cause of the electric 
spark vary ? Experiments showing this ? 



ELECTRIC ILLUMINATION. 



191 



Experiment a. — Place some eggs in a stand, as seen in 
Fig. 157. Fig. 157, connecting the bottom of the series 
with the earth, while the ball at the tojD is made 
to communicate with the prime conductor or a 
charged jar. The passage of the electricity 
through these will so powerfully illuminate each, 
as to show distinctly the whole ulterior struc- 
ture of each egg. 

163. If letters and other characters be formed 
by sticking with varnish small spots of tin foil 
on some insulating surface, as oiled silk or glass, 
so that each spot shall be separate from its neighbors, and an 

Fig. 158. 





electric spark be passed through these, the instantaneous pas- 
sage of the fluid will render the whole 

Fisr. 159. ^ 

word or other figure beautifully lumi- 
nous. 

Experiment. — Connect one end of 
the Luminous Frame^ Fig. 158, with 
the floor and earth; bring the ball at 
the other end to the prime conductor, 
or, which is better, to the outer ball 
of Lane's Discharger, properly adjust- 
ed, when the whole word will appear 
as above stated. A pane of glass set 
in a frame, and spotted to represent 
a profile or star, Fig. 159, presents, 
also, a splendid illumination when the 
electric spark is sent over it. 




Give the experiment with the 
rendered luminous by electricity ? 



eggs. How may letters, figures, etc., be 



192 



ELECTRIC ILLUMINATION. 




Experiment a. — Fig. 160 shows a glass tube spotted on 
the inside in a spiral form. Pre- Fig. lei. 

Fig. 160. ■* "^ . . - 

sent the ball of this to the excited prime 
conductor, as in the last experiment, 
holding the bottom by the hand, or con- 
necting its brass cap with the earth by a 
chain, and brilliant spirals of electric 
light Avill flash down the tube as the 
machine is worked. A string or neck- 
lace, formed by sew^ing together spots of 
tin foil with silk thread, may be ren- 
dered luminous in the same manner. 
fs%i8 Fig. 161 shows several of these 
tubes arranged on a circular stand, and 
illuminated by the revolution of two balls, 
attached to a wire lever, which revolves on a point upon the 
top of an insulated support. The wire and balls 
connect with the prime conductor, and, as they 
revolve, discharge the electricity from this, down 
the tubes, forming a most brilliant experiment. 
Experiment b. — Connect the knob of the 
luminous or Diamond Jar^ Fig. 162, with 
the excited prime conductor, and make a free 
communication between the outside coating and 
the earth. The electricity, as it distributes it- 
self over the inside of the jar, w^ill present one 
series of illuminations, and, as the same is re- 
pelled from the outside, another; and, thus 
constantly leaping across the insulated spaces, 
will render the whole jar luminous by constant 
flashes of light. 
164. Electricity passes xoith increased facility through 
rarefied air, the light diminishing in brilliancy as the rare- 
faction approaches a vacuum. 



Fig. 162. 




Give the experiment with the Diamond Jar. What is said of the passage of 
electricity through rarefied air ? Give Experiment a, with the spotted tube. 



ELECTRIC ILLUMINATION. 



193 



Fig. 163. 




Exj^eriment. — Screw to the plate of the air-pump, and 
also to the end of the sliding-rod, sets of 
metallic-points, as seen in Fig. 163. Con- 
nect with the prime conductor, work the 
Electric Machine, and at the same time ex- 
haust the bell-glass. As the air in this 
becomes more rare, the electricity will pass 
more freely between the sets of points, ex- 
hibiting a most singular luminosity, resem- 
bling the Aurora. 

Experimejit a. — Remove the sets of 
points in the last experiment ; screv/ a ball 
to the lower end of the sliding-rod ; place 
under the receiver a Leyden Jar; bring 
the ball of the rod near that of the jar, and 
connect with the prime conductor, as in the 
last experiment. Charge the jar in this 
position, and then exhaust the receiver. As 
the rarefaction proceeds, a luminous current of electricity will 
flow over the jar from the positive to the negative side, until the 
equilibrium is restored. 

Experiment b. — Place two metallic points within the long 
Guinea and Feather Tube (see Pneumatics, Fig. 50), and 
screw this by the stop-cock to the plate of the air-pump. Con- 
nect the ball of the upper end with the prime conductor, and, 
when the tube is well exhausted, work the machine. The 
electricity will now pass freely through the whole length between 
the two points, presenting streamers of light, strikingly similar 
in appearance to the Aurora Borealis. 

Experiment c. — If, instead of the points in the last experi- 
ment, two small balls be substituted, and, when the tube is well- 



Give the experiment -witli the sets of luminous points. Give Experiment o. 
Give the experiment for showing the luminosity by electricity by means of the 
Guinea anft Feather Tube, Fig. 50. What does the light in this experiment 
strikingly resemble ? 

17 



194 ELECTRIC ILLUMINATION. 

nigh exhausted, a charge from a highly electrified battery be 
passed, the electricity will descend in a brilliant ball of white 
light, forming the beautiful experiment known as the falling 
star. 

A beautiful modification of these experiments is shown by an 
apparatus, known as the Abbe Nollet^s Globe^ Fig. 164. A 

Fig. 164. 





glass globe with a long neck passing up through the opening 
of a glass receiver, is sealed to this, so as to be air-tight. Upon 
the top of this neck is a movable cap and ball, with a chain 
attached, and leading down into the globe. 

Experiment d. — Place the receiver on the plate of the 
air-pump, raise the cap and ball, and fill the globe with 
water as high as indicated by the cut. Connect the ball with 
the excited prime conductor, and at the same time exhaust the 
receiver. As the rarefaction of the air increases, streams of 
electric light will shoot across from the glass to the brass plate, 
constantly varying in color and form, with the degree of the 
exhaustion, from the white spark to the purple brush. 



Describe the Abbe Nollet's Globe. What is the experiment withithe Abbe 
NoUet's Globe? 



COMBUSTION BY ELECTRICITY. 



195 



COMBUSTION BY THE ELECTRIC SPARK. 

165. As we have already remarked, light and heat are the re- 
sults of the electric spark in its passage through such media as air 
and the gases, which offer a resistance to its course. These 
vary with the quantity of the fluid and the density of the medium 
through which it moves. Even with the limited quantity which 
may be collected in a Leyden Jar, or ordinary battery, the 
calorific effects of electricity may be shown by a great variety 

of curious and 
^^' "^ ■ striking illustra- 

tions. For many 
of these, the Uni- 
versal Discharger, 
Fig. 165, is a 
convenient instru- 
ment. 

Experiment. — 
Place on the in- 
sulating glass 
stand of the Uni- 
versal Discharger a ball of cotton filled with finely pow- 
dered resin. Bring to each side of this the balls of the sliding- 
rods ; connect one of the rods with the outside coating of a 
charged jar by a chain, and on the other place one ball of the 
discharger. Fig. 165, and bring the other ball quickly to the 
knob of the jar ; the passage of the electricity through the 
cotton will cause it to be inflamed. The same may be shown 
by placing the cotton on one of the balls of the discharger. 
Fig. 183, and bringing near the knob of the charged jar. 
Experiment a. — Reverse the sliding-rods of the last exper- 




How do the light and heat produced by electricity vary ? Give the experi- 
ment for inflaming cotton by electricity. What is the experiment with gun- 
powder ? 



196 



ELECTRIC BOMB. 



Fig. 166. 



iment, so as to bring the points within a half an inch of each 
other, while resting on the glass stand. Pour some fine gun- 
powder between these, and discharge as in the previous experi- 
ment; the powder w^ill be scattered in every direction, but not ig- 
nited. Cover the points with this as before, and let a wet string or 
body of water form a part of the connection between the sliding- 
rod and jar ; discharge again, and the powder will be readily 
exploded. This is probably occasioned by the surprising velocity 
with which the fluid travels (576,000 miles per second), not 
allowing, in the former case, sufficient time to produce its calor- 
ific efiects, and requiring for this a less perfect conductor, as 
water. 

166. Experiment. — The Powder Bomb, Fig. 166, exhibits 

a more striking form of the 
same experiment. Pour into 
this a thimble-charger full 
of fine powder; wet the 
string attached to one of its 
wires, and hook it on to one 
arm of the discharger ; let 
a chain lead from the other 
wire to the outside of a 
charged jar, and bring the other arm of the discharger to the 
knob of the same. The passage of the electric fluid across an 
interruption of the two wires will ignite the powder with a 
loud report.* Fig. 166 shows a sectional view of this instru- 
ment. 

* In all experiments for igniting gunpowder by electricity, a small glass 
tube filled with water forms the best connection. This may be about ten 
inches long, having two wires entering its ends, through tight stoppers, 
smeaxxd over upon the outside with sealing-wax. These wires should approach 
e \ch other in the tube, so as to leave about four inches of water between their 
ends. 




Why will this not be ignited by the electric spark when it is placed between 
good metallic conductors ? Give the experiment with the Powder Bomb. 



ELECTRIC CANNON. 



197 




Fig. 168. 



Experiment a. — Fill the 
Electric Cannon^ Fig. 167, 
with a mixture of hydro- 
gen gas and common air, by 
means of the Gas Generator, 
Fig. 168. Insert in this can- 
non a tight cork, and let a 
chain connect it with the outside of a charged jar. Pass the 
electricity down the insulated wire attached to the small ball of 
the priming-hole. As it passes from the end of this wire to the 
side of the cannon, through the gas, this 
will explode, and the cork be forcibly ex- 
pelled with a loud report. This cannon may 
be charged with powder also, and fired, as 
in the previous experiment. 

Experiment h. — A highly amusing 
form of the last two experiments, showing 
the igniting power of the electric fluid at a 
distance, may be- exhibited as follows : Let 
two small wires lead, unobserved, from the 
lecturer's table, along a side of the room, to 
a remote corner. Place there a loaded can- 
non, and let a glass tube of water form a 
portion of the connection, as remarked 
above. Place on the priming-hole a small 
ivory or wood cup. opening into the cannon, and provided with 
two wires with blunt ends, about an eighth of an inch separate. 
Connect one of these with the positive wire leading from the 
glass tube, and the other with the other wire, and fill the cup 
with fine powder. These may be arranged before the audience 
assemble ; and, during the lecture, a jar, placed upon the table 
before the lecturer, may be discharged through these wires, 
causing the cannon to explode, to the great astonishment of the 
unsuspecting comjDany. 

Wh.it experiment with the Electric Cannon ? 

17* 




198 MECHANICAL EFFECTS OF ELECTRICITY. 

A strong tin canister may be filled with explosive gas, and 
fired in the same unexpected manner ; the use of the glass 
tube, however, being unnecessary for this. The Galvanic 

Pistol, Fig. 191, Galvan- 
^'"■''^^' ic Electricity, may be 

arranged for firing by 
electricity. 

167. Experiment. — 
Pour into the EtherSpooii, Fig. 169, sufficient sulphuric ether, 
or strong alcohol, to cover its bottom. Hold this under a ball con- 
nected with the excited prime conductor, and let an electric spark 
pass into it, when the liquid will burst into aflame ; a common 
silver spoon will answ^er. A person electrically charged, as in 
Experiment c, § 151, may fire this by bringing over and near 
the liquid his finger, so as to allow of the passage of a spark. 

Experiment a. — Arrange two or three feet of iron chain in 
separate coils upon a sheet of white writing-paper, and pass 
through this a charge from about eight square feet of coated 
surface ; the portions of the paper upon which the chain rests 
will be scorched, and, if the charge be sufficiently great, even 
hnrned through^ by the heat of the electric fluid. 

Experiment b. — Place the hair-spring of a watch between 
the pincers of the Universal Discharger, and discharge through 
this a charge, as in the last experiment, when it id ill be literally 
consumed. Thus, lightning often melts metallic conductors, 
w^hich are too small to conduct it freely. 

168. Mechanical Effects of Electricity. — These effects are 
often seen in the passage of the fluid through imperfect conduct- 
ors, such being rent asunder with the greatest violence, without 
showing any signs of heat, as in the previous experiments. 

Experiment. — Suspend a thick card between the balls of the 
Universal Discharger ; bring these directly opposite and near the 

How may ether or alcohol be fired by the electric spark? Give Experiment 
a. Experiment with a fine watch-ppring? When are the mechanical effects 
of electricity exhibited ? 



DECOMPOSITION BY ELECTRICITY. 199 

same ; pass a charge from a battery, as in Experiment §1G5 ; the 
electricity will puncture the card without moving it, so great is 
its velocity. A peculiar hole is thus formed, with a burr pro- 
jecting out both ways^ as though the fluid had proceeded from 
the centre of the card. This experiment is regarded by many 
as one of the proofs of the existence of two fluids^ which, in a 
discharge between conducting bodies, proceed in opposite direc- 
tions. A still more convincing proof of this is given by 

Experiment a. — Color a card with vermilion, and place it 
between the points of the Universal Discharger, so that one of 
these shall be about half an inch above the other, and discharge 
the battery as in the last experiment. The card will be per- 
forated at the point connected witli the negative earth, while a 
black line of reduced mercury will trace the path of the elec- 
tricity along the surface of the card between the two points. This 
curious eifect is attributed to the greater rapidity with which the 
positive fluid passes through air ; for, if this experiment be 
performed in vaciio^ the perforation will always take place at a 
point iiitermediate between the two metallic points.* 

Experiment b. — Drill two holes in the ends of a piece of 
dry wood, half an inch long, and a quarter of an inch thick ; 
insert two wires in these, so that their ends shall be rather less 
than a quarter of an inch apart ; pass the charge of a large 
battery through these wires, and the wood will be split w^ith 
violence. The same may be done with stones. 

Experiment c. — Suspend in a common wine-glass, nearly 
filled with water, two wires, tipped with metallic balls, so that 
these shall be about half an inch apart. Send through these 
wires a charge from a four-quart jar or small battery ; the 

* Bird. 



Experiment ■with the card placed between the two balls of the Universal 
Discharger ? How is this experiment supposed to prove the existence of two 
electx'ic fluids ? State Expeinment a. To what are the curious effects seen in 
this experiment attributed ? State the experiment where an electric discharge 
is made through a wine-glass filled with water. 



200 



DECOMPOSITION BY ELECTRICITY. 



passage of the electricity through the water between the balls 
will cause the glass to be shivered. 

Large rocks are sometimes removed by drilling a hole and 
insertuig the lower end of a metallic rod which shall attract the 
lightning. This, passing down the rod, enters the rock, scat- 
tering it with a force far superior to gunpowder. 

169. Decomposition by Electricity. — That form of electricity- 
most effective in causing the decomposition of compound sub- 
stances is produced by chemical action, and known as Galvan- 
ism. This is characterized by a far lower tension, but a vastly 
greater quantity, than common frictional or static electricity. 
The decomposition of a liquid, for instance, depends on the 
quantity of electricity passed through it ; hence, for rapid de- 
composition, the Galvanic Battery is commonly employed. The 
same may, however, be effected to a certain extent by the elec- 
tricity from friction. 

Experiment. — Remove one of the screws and rod from 
the end of the thick glass receiver of the Decomposing Appa- 
ratus^ Fig. 170, and fill this with water. Replace, and bring the 

Fig. 170. 




two balls near each other ; pass repeated charges from a large 
battery through the water between the balls ; the gases oxygen 
and hydrogen, the elements of which the water is composed, 

How are large rocks sometimes rent asunder by the aid of electricity ? 
What form of electricity is most effective in producing the decomposition of com- 
pound substances ? Can such decomposition be effected by common frictional 
f U'ftrlcity ? Give the experiment for illustrating this. 



L>ECOMPOSITION BY ELECTRICITY. 



201 



Avill be separated and collected in the upper part of the receiver. 
If a sufficient number of discharges be made, these gases ^Yill 
be produced in such quantities as to fill the receiver down to 
the balls, when they will be ignited bj the electric spark, and 
reunite to form water again, this rising and filling the receiver 
once more ; thus showing the decomposition and recomposition 
uf water by the electric sptark. 

Expei^iment a. — Drop into the Electric Mortar^ B, Fig. 

171, sufficient oil or water, 

©^iL. '^ ^ to cover the ends of the con- 

T^^***H;^ j^^^ ducting-wires. Place the 

^J. ^^ A ^^^^^ ^^11? <^V6^ the hole, to which 

it is nicely fitted, and pass 
the electricity from a 
charged jar, as shown in 
the cut. As it passes be- 
tween the wires, through 
the liquid, this will be de- 
composed^ and form gases, 
which will suddenly expand, 
so as to throw the ball sev- 
eral feet into the air. K 
repeated discharges from an electric battery be made through a 
portion of confined air, the two gases, oxygen and nitrogen, 
v;hich are mechanically mixed to form this, will become chemi- 
cally united by the electricity, and form an exceedingly strong 
acid, known as aqua fortis. Thus, lightning acts to coagulate 
or sour milk and other albuminous liquids, by forming in the 
atmosphere, during a thunder-shower, this corrosive acid. 

170. Electricity is strongly induced about metallic points, 
which attract and discharge it with far greater facility than 
blunted surfaces. 




Give the experiment with the Electric Mortar. The effect of repeated dis- 
charges of electricity through a portion of confined air ? Cause of the souring 
of milk, etc., after a thunder-storm? Effect of metallic points in reference to 
the electric fluid ? 



202 ELECTRIC AURA. 

Experiment. — Place a pith-ball electrometer on the knob 
of a jar, and charge positively, or on tlie prime conductor. If, 
now, a metallic ball be held toward this, it will have but little 
influence in removing its positive electricity, as shown by the 
very gradual descent of the pith-balls ; but remove this, and 
present instead a pointed wire, when the electric fluid will be 
rapidly withdrawn, and the balls quickly fall. If in a darkened 
room, the point held toward a positively charged body, as the 
Leyden Jar, in this instance, w^ill show a star of electric light, 
and, if presented to a negatively charged body, a brush of light. 
It is thus, that, during the passage of a highly electrified 
cloud, spires of churches, masts of vessels, and other pointed 
objects, are often, during the night-time, tipped with light, the 
electricity passing between the charged cloud and pointed object, 
the same as between the prime conductor and metallic point. 
Such a light is known among sailors as Castor and Pollux, and 
is a cause of dread, as oftentimes immediately preceding a dis- 
charge of lightning. 

171. The Electric Aura is a current produced in the air by 
the escape of electricity from a 
point. This is often sufficient to 
cause small wheels to revolve, and 
even to move delicate machinery. 

Experiment. — Place a pointed 
bent wire in the ball of the prime 
conductor, and bring near it a del- 
icate Jloat-icheel^ as seen in Eig. 
172. Work the machine, and the 
escape of the electricity from this will 
cause the wheel to revolve rapidly. 

Ho-w shown by a metallic point placed on the prime conductor ? When will 
the illumination at the point be a star ? When a brush of light ? What is 
said in regard to the phenomenon occasionally exhibited during the night- 
time by pointed objects when electrified clouds are passing ? What are 
these lights called by sailors ? What is the Electric Aura ? How shown by 
the float- wheel and point ? 




ELECTRIC SEASON-MACHINE. 



203 



Fig. 173. 



The reaction of the current thus formed may be well illus- 
trated by the following : 

Experiment a. — Stand the pointed wire, Fig. 
173, with its wheel of S's, on the prime conductor, 
and excite the machine. The escape of electricity 
will produce a reacting current, which will cause 
this wheel to revolve rapidly, and in the dark 
these points will present a beautiful luminous 
circle. 

Experiment h. — Provide such a wheel with an 
axis, and let the ends of this rest on an inclined plane of wires, 

as shown in Fig. 174. Con- 
nect with the excited prime 
conductor ; the reaction 
against the points will cause 
the wheel to roll up the in- 
clined plane. 
Experiment c. — A highly 
amusing illustration of electrical reliction^ may be shown by 
the Electric Season-Mac/mie, Fig. 175. Here an arrangement 

representing the sun, earth, 
and moon, is balanced on a 
point upon the top of an 
insulating post. From each 
of these bodies and the 
main wire project points. 
When electrified from con- 
tact with the prime con- 
ductor, the escape of the 
fluid from the points will 
cause a reaction sufficient to make both sets of wires, with their 
balls, revolve somewhat after the astronomical order. 




Fig. 175. 




How may the reaction of the air on a current of electricity escaping from 
points be shown ? Explain the operation of the Electric Season Machine. 



204 



ELECTRIC SHOCKS. 



Fig. 176. 




Electricity is an efficient agent in promoting evaporation^ 
"\yhich it does by its force of self-repulsion, 
driving asunder the particles of liquid, and 
thus rendering them minute and volatile. 
This may be well illustrated by an arrange- 
ment seen in Fig. 176. 

Experiment d. — Suspend from the prime 
conductor a small metallic bucket of water ; 
place in this a siphon, and cause it to flow, 
and work the machine. The electrified water, 
instead of falling m a stream, will be dispersed in a fine mist. 
172. Effects of Electricity on the Animal System. — 
Whenever the body of an animal is made a part of the electrical 
circuit, an involuntary and painful muscular contraction is 
experienced ; and, if the charge sent through this be sufficiently 
great, instant death is the result. If a powerful charge be 
passed through a portion of the body, as a single limb, for 
instance, the sensibility of this may be destroyed. Thus, Van 
Marum found that the eel, an anjmal exceedingly tenacious of 
life, was instantly killed by a shock passed through the whole 
body ; and when passed through only a portion of this, that 
portion was destroyed. 

E.Tperiment. — Place any animal, as a rat, for instance, in 

the glass tube, Fig. 
177, and discharge 
through it a large jar 
or battery — instant 
death will be the re- 
sult* 

*The bodies of animals killed by lightning are found to undergo rapid 
putrefaction, and it is a remarkable circumstance that after death the blood 
does not coagulate. 



Fig. 177. 




What relations is electricity supposed to sustain to evaporation ? How docs 
it promote evaporation? Give the experiment "with the small bucket and 
siphon. State the cflfects of electricity on the animal system. Case of the col. 



MEDICAL ELECTRICITY. 205 

Experiment a. — Let any number of persons join hands ; 
charge a quart jar, and while the one at one end clasps the 
outside of the jar, let the one at the other end complete the 
electric circuit, by touching its knob, when the whole will ex- 
perience a smart shock in the arms at the same instant. Thus, 
Abbe Nollet passed an electric charge through a circuit of fifty- 
four hundred persons, when a convulsive shriek was given at 
both ends at the same instant. 

Experiment b. — An amusing illustration of the same prin- 
ciple may be shown as follows : Coat a large pane of glass with 
tin foil to within about two and a half inches of its edge, on 
both sides. This may be set in a wood frame, similar to Fig. 
159 ; now connect the coating of the under side, by a narrow 
strip of tin foil and a chain, with the earth ; place upon the other 
side a piece of coin, and connect with the excited prime 
conductor ; the glass will become charged like a Leyden Jar, 
and, if attempts be made to remove the coin with the fingers, 
the person, w4io will form a portion of the electric circuit, 
will receive a painful shock, causing a failure in the attempt. 
This forms the well known experiment with the Miser^s 
Plate. 

173. The virtues of electricity as a medical agent were, in 
the early stages of the science, greatly exaggerated. Electrified 
medicines, purporting to efiect remarkable cures, were dealt to 
the patient in medicated tubes, and a variety of gross imposition 
was for a time practised by learned pretenders. Such a course 
soon brought electricity, as a healing agent, into disrepute, and 
caused it to be well-nigh discarded in medical practice. Its 
efficiency, however, in cases of paralysis and imperfect circu- 



How may a company of persons be arranged for receiving the shock from a 
Leyden Jar? Give the experiment with the Miser's Plate. How were the vir- 
tues of electricity, as a medical agent, regarded in the early stages of electrical 
science ? In what diseases of the body is it beneficial ? What kind of elec- 
tricity is usually employed for medical purposes ? 

18 



206 MEDICAL ELECTRICITY. 

liition, is now too well known to admit of a doubt.* The kind 
of electricit}^ more generally administered is that of the galvanic 
battery. The form of instruments, and mode of application 
to the patient, will be found fully described in w^orks specially 
devoted to this subject. 

* Ferguson, in his Introduction to Electricity, published in 1778, relates 
the following among many other remarkable cures through the agency of 
electricity. We give them in his own words. " A young man, who had well- 
nigh lost his hearing, so that those who spoke to" him were obliged to speak 
very loud, came twice to be electrified. I only drew sparks from his ears, and 
at the second time he heard very well, and continued to do so afterwards. 

" I was once, at Bristol, seized with a sore throat, so that I could not swal- 
low. Mr. Adlam, of that city, who is a fine electrician, came and drew many 
electric sparks from my throat, and, in about half an hour after, he did the 
s ime again. He staid with me about an hour longer, and before he went 
away I could eat and drink without pain, and had no return of the disorder. 
I have relieved several persons in such cases, but never in so short a time. I 
have often drawn sparks from chilblains, and always found they were cured 
thereby. 

" One time my -yvife happened to scald her wrist by boiling water. I set 
her upon the insulating stand directly, and took sparks from the wrist. In a 
short time I found the redness of the skin begin to disappear, and she felt 
immediate relief. I repeated the operation, which entirely cured her, and 
there Avas not the least blister left on the skin." 



ATMOSPHERIC ELECTRICITY. 207 



ATMOSPHERIC ELECTRICITY. 

174. The atmosphere is always found charged, to a greater or 
less extent, with electricity. This varies, in kind and degree, 
with the causes which produce it. As we ascend to the more 
elevated regions, the quantity of electricity present in the 
atmosphere increases; and, in clear and settled weather, this 
is found to be positive in regard to the earth. In changeable 
weather, a,nd especially during a thunder-storm, the atmosphere 
is sometimes positive and sometimes negative, often changing 
its electric state quite suddenly. The presence of electricity 
in the atmosphere may be readily shown by extending into 
it a pointed metallic rod, insulated, to the bottom of which is 
attached a delicate electrometer. Even in clear weather suffi- 
cient electricity may be attracted by this to cause the leaves 
of the electrometer to diverge, and, during a thunder-storm, 
vivid sparks can often be taken, sufficient to charge jars and 
electric batteries, and perform all the results of a common 
electric machine. 

175. We have already, in the introduction, alluded to the grand 
discovery by Franklin of the identity of lightning and com- 
mon machine electricity. This he proved, most conclusively, 
by means of a kite, which he elevated during the passage of 
a thunder-cloud, when the electricity or lightning from this cloud 
passed down the wet string, and was collected in jars, like 
that from an ordinary electric machine. 

176. By means of an extended and connected series of pointed 
wires, erected on trees and elevated objects, a prodigious 
quantity of electricity may be collected for experiment; far 

What is said of the electricity present in the atmosphere ? What is said of 
the electric state of the atmosphere in changeable weather, 'and during 
thunder-storms ? How may the presence of electricity in the atmosphere be 
detected ? Experiment of Franklin for proving the identity of lightning and 
electricity? How may large quantities of electricity be collected from the 
atmosphere ? 



208 ATMOSPHERIC ELECTRICITY. 

exceeding that produced from any artificial source. Thus, Mr. 
Crosse, a celebrated English electrician, was enabled, with 
one third of a mile of wire, to collect from the atmosphere, 
when in a highly electrified state, sufiicient electricity to 
charge a huge battery, of seventy -three square feet of coated 
surface, " twenty times in a minute^ accompanied by reports 
as loud as a cannon." And with a large kite, having a string 
inwoven with fine wire, and elevated to the height of five 
hundred and fifty feet, M. de Komas, a French philosopher, 
obtained from the conductor to which the string was attached 
during the passing of a thunder-cloud, "balls of electric fire 
ten feet in length, and an inch in diameter." 

177. Evaporation is regarded as the prime agent employed 
in distributing electricity through the upper regions of the atmos- 
phere. The earth is the great reservoir for the electric fluid ; 
from this the vapors, which are constantly rising, bear it aloft, 
these being rendered volatile by the self-repellant properties 
of the electricity which they contain. When this watery 
vapor reaches an elevation where the condensing power of 
cold is sufficient to overcome the repulsive force of the elec- 
tricity, clouds are formed, and the electric fluid, like latent 
heat, becomes sensible in the condensed vapor of the cloud. 

178. Thunder- Clouds^ of all aerial bodies, exhibit the 
highest degrees of electric excitement. These are usually 
formed, during the heat of summer days, from the rapid 
condensation of vapor with which the atmosphere is then satu- 
rated. As they increase in density and extent, the tension 
of their free electricity becomes greater, until soon this reaches 
such a degree as to overcome the resistance of the non-con- 
ducting air, when a discharge of lightning takes place, either 
towards some neighboring cloud less electrified, or the earth. 

Describe the apparatus of Mr. Crosse for collecting electricity from the 
atmosphere. Give the result of M. de Romas' experiment. What is said of 
the agency of electricity in evaporation ? How does electricity aid evapora- 
tion ? AVhcn are thunder-clouds usually formed ? How is lightning evolved 
from these, and upon what does it discharge itself? 



THUNDER-CLOUDS. 209 

Isolated clouds, which form rapidly from vapor condensed 
by the meeting of warm and cold currents of air, usually 
present the most terrific electric phenomena. Such are usually 
marked by a violent commotion, a flying and whirling of 
detached fragments about the denser portions, a black and 
threatening appearance, and, as they increase in size, by 
frequent discharges of lightning, followed by terrific peals of 
thunder and torrents of rain. 

179. The thunder-cloud may be regarded as a huge electric 
battery suspended in the heavens, and insulated by the sur- 
rounding air. In this prodigious quantities of the electric 
fluid accumulate, and, when overcharged, discharge, like the 
ordinary battery, upon the nearest object. =^ These clouds are 
often negative in regard to each other and the earth ; in such 
a case, the lightning is seen to leap from one to the other, 
until the equilibrium between them is restored. Occasionally 
flashes of lightning are seen to pass between two clouds, sepa- 
rated by a considerable distance, and the earth, at the same 
moment. In such instances, the clouds are supposed to be 
in widely difierent electric states, but too far separated to allow 
of a direct passage of the fluid, which takes a circuitous route, 
passing from the positively electrified cloud to the earth, and 

* The quantity of the fluid in a discharge of lightning varies with the 
dimensions of the cloud, and the tension of its electricity. The hole forced by 
lightning in its passage through solid bodies varies in size from that formed 
by a small rifle ball to one foot or more in diameter. The following descrip- 
tion of the recent effects of lightning on a dwelling-house, upon the island 
of Great Chebeaque, Maine, was furnished the Portland Advertiser, by an eye 
witness, and serves to show the prodigious quantity of electricity which occa- 
sionally passes from a highly charged cloud. 

" The lightning appeared as a ball of fire, apparently a foot in diameter, with 
a trail some thirty yards in length. This passed down the chimney, shatter- 

What kind of clouds generate electricity most rapidly ? Give the appear- 
ance and phenomena exhibited by such clouds. Correspondence between a 
thunder-cloud and a Leyden battery ? Course of the electric fluid between 
two clouds, in different states, but separated by a wide interval ? 

18* 



210 



RETURN STROKE. 



then from this to the negative cloud ; the instantaneous pas- 
sage of the fluid causes the lightning, in this case, to appear to 
descend to the earth from both clouds at the same instant. 
Oftentimes thunder-clouds, highly electrified, are seen to break 
upon the sides of mountains, or the surface of the ocean, Avhen 
the discharges of lightning are directed Tvhollj to the earth, 
and become frequent and violent in the extreme. 

180. We have already shown the tendency of electricity 
to repel its like, and attract its opposite kind. Thus, when a 
cloud, A B, Fig. 178, highly charged, say with positive elec- 
tricity, is suspended 
over and near the earth, 
it acts by induction, 
through the dielectric 
air, to repel the same 
electricity from that 
portion of the earth 
directly beneath, and 
attract its opposite. 
When such a cloud 
discharges, the return 
of the repelled fluid 
to the surface again sometimes produces a shock sufficient to 
destroy life. This is known as the return stroke^ and by it 




ing the house in a fearful manner, but not firing it. Nearly every article, of 
crockery was broken in pieces ; and two clocks, three looking-glasses, and 
two heavy oak tables were destroyed. Every chair in the kitchen was broken, 
and eveiy partition removed from its proper position, and more or less shat- 
tered. The chair upon which the lady of the house was sitting was broken 
in twenty-eight pieces. The hole formed by the passage of the lightning 
through the floor into the cellar was sufficiently large for the passage of a 
man. A stone arch in the cellar, two feet thick, was rent asunder, in four 



Retults when thunder-clouds sometimes break upon mountains, or the sur- 
face of the ocean? Explain, from the figure, how a person may be killed by a 
discharge from a cloud when several miles distant from tlie lightning. V,' ii.it 
io this form of stroke called ? 



LIGHTA^ING-RODS. 211 

a person on an elevation, at D, say, may be killed, although 
ten or twenty miles distant from the lightning of the cloud. 

181. Lightni)i(j-rods are a comparatively recent invention, 
being one of the results of the practical investigations of Dr. 
i'ranklin. We have before shown the power of metallic points 
iii attracting the electric fluid, and withdrawing it from elec- 
trified bodies. The phenomena of these suggested to the mind 
of that acute philosopher the idea of pointed metallic rods 
raised upon buildings, for protecting these against the dangers 
of lightning. These may be made from three-quarter inch 
nail-rod (although copper is preferable as a conductor), ex- 
tending up from the chimney, and from the ends and corners of the 
roof, from three to six feet, and terminating in points ; * the 
whole being connected with one or two main rods, conducting 
to the earth. These should enter the ground sufficiently deep 
to be always in contact with a moist earth ; and may terminate 
in some powdered charcoal, or a spring of water. The space 
which a point will protect is much less than is often supposed ; 
this being a sphere w^hose radius is twice the height of the 
point above the building. Thus, a point rising five feet above 
the roof will protect a spherical space of twenty feet in 
diameter ; hence, the necessity of several of these upon a large 
building. The rods should be as entire as possible, and the 
stays used in holding them, if of metal, should not terminate 
in points near other metals within the frame of the building ; 

places, and the cellar-wall twice severed from top to bottom, and the stones 
blackened as if by powder. Every window-sash in the house, with one 
exception, was demolished ; five doors were shivered to splinters. A large 
trunk, filled with clothing, was subsequently opened, and its contents found 
to be covered with soot to the depth of half an inch. Several trees, at a dis- 
tance of six rods from the house, were shattered in pieces ; but, remarkable to 
say, amidst this general wreck, the inmates of the building escaped with life." 
* Silver points are preferable. 



What is said in regard to the discovery of lightning-rods ? How may they 
be made ? How should they be placed upon a building? What space around 
will a pointed couJuotur commonly protect ? 



212 



THUNBER-HOUSE. 



Fig. 179. 



as, in case of an excess of lightning passing down the rod, a 
portion may pass along the stay into the building. 

Some amusing experiments, illustrative of the uses and 
defects of lightning-rods, may be shown, as follows : 

Experiment. — Place upon the Thunder-House, Fig. 179, 
either form of points. Without 
the metallic chain leading within, 
as shown in the cut, join the parts 
of the rod so as to make it en- 
tire, and connect the bottom, by 
a chain, with the outside of a 
charged jar; bring this jar near 
the point, when it will be silently, 
yet rapidly, discharged. Place a 
^™^ ball on the point, and the jar will 
now discharge with a flash, yet 
without harm to the building. 

Experiment a. — Place, now, within the same, a small 
canister of explosive gas, as described in Experiment §166; 
connect this with the rod upon the inside, as shown in the cut, 
and bring the chain, connected with the metallic surface on 
which the canister rests, to the outside of a charged jar ; 
break the connection of the rod a trifle, and bring the knob of 
the jar near the ball or point. The fluid, instead of passing 
as before, will be diverted into the building, exploding the gas, 
and scattering the parts of the house in every direction ; thus, 
strikingly illustrating the disastrous effects which frequently 
proceed from a defective lightning-rod.* 




* The parts of this miniature house are held together by magnets. One side 
of the roof may be removed for arranging the apparatus within. The canis- 
ter, if tight, may be filled and adjusted before the lecture ; in such case, the 
order of these experiments must be reversed. Powder may be substituted, by 
connecting with the rod by a wet string. 



State the experiment with the thunder-house? Give Experiment a, illus- 
tr.iting a defective lightning-rod ? 



LIGHTXi:;^G — PLACES OF SAFETY. 213 

182. Thunder is caused bj a sudden concussion of the air, 
Tvhicli has been forced aside bj the electric fluid in its passage 
through this ; the Tolling sound is produced by the rever- 
beration among the clouds, and the return of the report from 
the varying distances along the electric line. The zig-zag 
course of lightning is, probably, caused by the condensation oi 
the air before the fluid, which turns it aside from a direct line. 
Vapors, however, and different conducting media, may some- 
times determine its course from a straight line. 

183. Places of Safety. — The sudden and terrific effects of 
lightning render it a cause of universal dread, which naturally 
leads to an inquiry in regard to the proper precautions against 
its dangers. The directions are few and simple. Lightning- 
rods, properly constructed, are, undoubtedly, important safe- 
guards against its effects in dwellings, and on board of vessels. 
As chimneys, charged with soot and smoke, are excellent con- 
ductors of the fluid, and very liable to attract it, a seat near a 
stove or fire-place, during a thunder-storm, should be avoided ; 
also, against bell-wires, gilt mouldings, or by an open window, 
as the electricity is most certain to take these in its course 
through a building. The cellar, also, is an unsafe place of re- 
sort ; the fluid often passing from the earth to the cloud, when 
the most disastrous results may be effected in the basement. 

Reclining upon a feather-bed, sitting in the centre of a car- 
peted room, or standing upon a thick rug, are tolerably safe 
positions. 

If out of doors, trees, especially those standing near bodies 
of water, should not be sought for shelter ; but rather cliffs of 
rocks, low sheds, caves, etc. The chances, however, of death 
from lightning are exceedingly few, less even than from the fall 
of chimneys, the upsetting of carriages, or a thousand similar 
liabilities, which seldom occasion the least solicitude. 

Ho-w is thunder caused ? Cause of the zig-zag course of lightning ? What 
are some of the dangerous positions during a thunder-storm ? Some of the 
more safe positions ? 



214 AURORA BOREALIS. 

184. Heat- Lightning is probably a reflection of the flaslics 
of lightning produced by storms actually below the horizon. 

185. The Aurora Borealis is undoubtedly the result of an 
electric agency. Various theories have been entertained in 
regard to the probable cause of this singular phenomenon ; but 
nearly all unite in ascribing this in some way to the electric 
fluid. De La Rive, a philosopher, whose opinion on such sub- 
jects is entitled to much regard, explains the cause of the 
Aurora as follows : 

The constant escape of positive electricity from the earth, by 
evaporation, renders the upper atmosphere highly positive, and 
there is consequently a continual tendency to restore the equi- 
librium between this and the earth. Although, by electrical 
discharges from clouds, and by the fall of rain and snow, this 
is partially efiected, yet the main channel, through which the 
fluid passes to the earth, is found to be at the magnetic poles. 
Thus, the quantities of electricity which pass into the atmos- 
phere, about the equatorial regions, flow towards the poles, and 
are discharged into the earth at the extremities of the mag- 
netic axes, and from thence flow again to the equator, thus 
forming a constant and continuous electric circuit through the 
upper atmosphere toward the poles, and through the earth to- 
ward the equator. 

The streamers of auroral light are caused by the passage of 
electricity in a circle, from the regions above the magnetic polo 
toward this pole. In demonstration of this theory, De La Rive 
caused one of the poles of a straight magnet in connection with 
the earth to enter a glass globe. Around and above the end 
of this magnet was fixed a metallic circle, in communication 
with an electric machine. Upon removing the air from the 
glass globe by means of the air-pump, and charging the me- 
tallic circle from the electric machine, the fluid was found to 

IIow is heat-liglitening probably caused ? By what agency is the Aurora 
Borealis probably produced? Give the experiment of De La Rive, for illus- 
trating the cause of the Aurora. What evidence have we that the Aurora id 
connected with the magnetism of the earth ' 



AUROEA BOREALIS. 215 

pass from all parts of the ring towards the pole of the magnet, 
forming streams of dim light, and a halo resembling with much 
precision the phenomenon of the Aurora. 

That this is connected in some way with the magnetism of 
the earth is evident from the fact that the magnetic needle is 
found to be more or less affected by north and south currents 
during auroral exhibitions. 

The Aurora often affects most sensibly the operation of the 
Electro-Magnetic Telegraph, causing the register of this to 
operate as if connected with a powerful battery. The irregular 
action of the telegraph during the day sometimes gives the 
signal of an Aurora, which is verified by its appearance at 
evening. 

The affection by the Aurora is most sensible in Bain's Chem- 
ical Telegraph. Thus, by its effects upon the telegraph, this 
illumination is shown to be connected in some way with the 
electric agent. 

What is said of the Aurora in reference to the Electro-Magnetic Telegraph ? 



216 GALVANIC ELECTRICITY. 



GALVANIC ELECTRICITY. 

186. OccuRRENCESj the most trivial in themselves considered, 
are often suggestive of principles the widest and most impor- 
tant in their application. In about the year 1789, Galvini, a 
professor of Anatomy, at Bologna, had his attention one day 
arrested by the following singular phenomenon. Some frogs' 
legs, prepared for a soup for his wife, who was an invalid, were 
suspended by copper hooks connected with an iron railing. As 
these were moved by the wind or other cause so as to touch the 
iron, they were noticed at the same instant to become convulsed, 
and exhibit a peculiar twitching movement, as if possessed of 
vitality. Galvini at once commenced a series of experiments, 
and produced the same phenomenon, in a more marked degree, by 
the application of various metals to the nerves and muscles of 
the legs of frogs freshly prepared ; and hence, from so slight a 
circumstance, was laid the foundation of that branch of phys- 
ics, named from its discoverer Galvanism; a science, which 
has of late contributed in such a wonderful degree to the pro- 
gress of art, and to enhance the physical and social well-being 
of community. 

Galvini attributed these movements of the muscular system 
to a nervous fluid, which passed from the nerves to the mus- 
cles, to restore an equilibrium whenever the metals connecting 
with these were touched together, producing at the same time 
a convulsive shock, similar to the attempt to restore the dis- 
turbed equilibrium of a charged Leyden Jar. 

187. Volta, a distinguished electrician, soon after instituted 
a series /)f experiments, to refute the theory of Galvini, and 
showed that no electrical or nervous excitement took place, un- 



State the circumstances connected with the discovery of Galvanism. What 
is said of the importance of this branch of science ? To what did Galvini at- 
tribute these singular movements of the frogs' legs ? What did the experi- 
ments of Volta show ? 



GALVAXIC ELECTRICITY. 217 

less the metals connecting the parts of the animal -were of dif- 
ferent kinds, as iron and copper, or copper and zinc ; he was 
hence led to refer these singular results to the development of 
free electricity by a contact of the metals. Thus, in the case of 
zinc and copper, when in simple contact, free positive electricity 
was found to be evolved from the former, and free negative 
electricity from the latter. These Yolta attributed to a pecu- 
liar electro motive force, under which metals by simple contact 
tend to assume different electrical states. 

The late experiments of De La Rive, Faraday, and other 
distinguished philosophers, have overthrown the theory of Yolta 
also, and proved this form of electricity to be in every instance 
the result of chemical action. 

The discovery of the Voltaic Pile by Yolta, in about the year 
1800, served to establish, beyond a doubt, the identity of elec- 
tricity and galvanism, and gave a new phase to this wonderful 
branch of science. 

18-8. When two metals.^ differing in their suscejotihility of 
oxidation, are connected and placed i?i some liquid capable 
of acting more powerfidly on one than on the other, that 
form of electricity known as galvanism or electricity in mo- 
tion is produced. 

Experiment. — Pour into the glass vessel. Fig. 180, some 
dilute sulphuric acid, and in this pla<3e two 
metallic plates, one of zinc and the other 
of copper, with wires attached as in the 
cut. If, now, the wires be joined, a gcdvanic 
s^ IJh— i-J L circle will be formed; the water of the 

^^^^~— ^.^ ^^^ liquid becoming decomposed,* its oxygen 
will unite with the zinc (the more oxida- 

* Water is a compound, formed by the union of the two simple gases oxy- 

To what was he led to refer these results ? What is said of the late experi- 
ments of De La Rive, Faraday, and others? What did the discovery of the 
Voltaic Pile serve to establish ? State the proposition section 188. State the 
experiment illustrating this. 

19 




218 GALVANIC ELECTRICITY. 

ble metal), and at the same time a current of electricity will 
be transmitted through the liquid to the copper, on the surface 
of which hydrogen, the other element of water, will appear 
in the form of minute bubbles of gas ; this current of elec- 
tricity passes around through the wires in the direction indicated 
by the arrows, and returns again to the zinc where it originated ; 
and thus a constant electric current is maintained, so long as 
the wires are made to touch ; but separate these, and the flow 
instantly ceases, and so continues until contact is again made. 
Such an arrangement constitutes a Simple Galvanic Battery. 
If a series of these metallic plates be made to alternate, each 
copper being joined to its corresponding zinc, as shown in Fig 

181, it forms the Voltaic Pile or Com- 

^^L^^ pound Battery. 

2(O^^C^X4C~Xir\c ■^^^- ^^^ copper plate, or extremity 

Pfn rril iFn I at which the electricity passes /;'o/;^ the 

Ij. — LzU J liquid, is termed the positive pole^ 

— ^ — while the zinc plate, or extremity where 

it enters ii again, is called the negative 
pole. These poles have also been styled, by Dr. Faraday, e/ec- 
^ro(/e5,^ or ways for the electricity ; the positive pole being 
called by him the a?iode,\ or ascending way, and the negative 
pole the cathode ;l or descending way, or path for the electric 
fluid ; these terms expressing more clearly the direction of this 
in its flow through the parts of the battery. 

190. It is not necessary, in order to form a galvanic circuit, 

gen and hydrogen. These are in ditferent electrical states, one positive, and 
the other negative. The poles of the Galvanic Battery having a stronger at- 
traction for these than they have for each other, they separate and go to their 
respective poles. The same is true of other compounds besides water. 

♦From the Greek, )\XBy.ronr, electricity, and ocTuc, -way. 

t avu, ascending, octoc, way. % /.arct, descending, o^ocy way. 

What does such an arrangement constitute ? What constitutes a Compound 
Galvanic Battery ? What is the positive pole ? What is the negative ? What 
were these poles styled by Faraday ? 



GALVANIC ELECTRICITY. 219 

tliat there be two different metals, or two kinds of liquids, pro- 
vided certain other conditions be attended to. Thus, if a single 
plate of zinc be so fitted into a wooden trough as to form of it 
two separate cells, and into one of these, upon one side of the 
metal, there be poured some dilute acid, and into the other, upon 
the opposite side, a solution of common salt, a current of elec- 
tricity will at once commence to flow through wires connecting 
the two sides of the plate ; or if the same liquid be used in 
both cells, and one side of the zinc plate be rough and the 
other smooth, a like action will result. Thus, in order to excite 
galvanic action, it is only necessary to produce different degrees 
of chemical action on different plates of metal, or on opposite 
sides of the same plate. 

191. Galvanic electricity is characterized by its immense 
quantity and continuous flow^ hut feeble tension ; frictional 
or machine electricity, by its limited quantity and irregular 
discharge^ but high tension, and the great energy of its me- 
chanical action. — The machine electricity, employed in the 
experiments of the previous sections we have shown to be pos- 
sessed of a remarkable degree of tension, which required the most 
perfect insulation, in order to prevent its escape from the sur- 
face of the bodies upon which it was desired to retain it ; and its 
quantity, although small, was characterized by a spasmodic, yet 
w^onderful energy of action. Galvanic electricity, on the other 
hand, is evolved in prodigious quantities, yet with a. steady and 
continuous flow, and may pass through conductors, but slightly 
separated from other conducting media ; thus, the wires which 
conduct away the electricity produced from a powerful battery 
require only the insulation afforded by a coat of varnish or 
cotton thread, while, in the case of electricity produced by me- 
chanical means, these conductors require a separation of a con- 
siderable distance. 



How may a galvanic current be produced by a single metal or a single liquid ? 
Distinction between galvanic and frictional electricities ? 



220 GALVANIC ELECTRICITY. 

The follawing illustration may, perhaps, best serve to convey 
an idea of the nature and effects of these two forms of elec- 
tricity. Suppose a reservoir for water to be situated upon an 
eminence, and supplied by a constant flow from an adjoining 
spring, and from this reservoir a pipe conduct to the region 
below. If this pipe be left open, so that the water may have a 
free and continuous passage from the reservoir, the quantity 
passing in a given time, although great, will not be suffered to 
accumulate, so that its force will be comparatively feeble and 
unobserved ; but if, now, this pipe be provided with a valve at 
its lower extremity, which shall remain closed, and open only 
as the reservoir becomes filled, and the intensity of the pressure 
from the accumulated water too great to be longer withstood, a 
violent bursting forth of the fluid will occur at stated intervals, 
accompanied by effects the most forcible and energetic. Thus, 
with a comparatively small quantity of liquid under a high 
pressure, more striking effects may be produced than with a 
very much larger amount unrestrained. So of the results of 
the electricities excited by frictional and galvanic action, while 
the former produces its effects through an accumulated and 
intensified action, the latter, flowing forth in a mighty yet con- 
tinuous stream, works its changes through its quantity^ rather 
than by any concentrated power. 

192. The tension of galvanic electricity may he in- 
creased^ and made to approximate to that produced by the 
decline machine. — This may be effected by increasmg the 
number of the pairs of metals employed in exciting the galvanic 
current. The quantity of electricity set in motion by a single 
pair of metallic plates is the same as that from a long series of 
the same, while the intensity of this varies with the number 
of the plates forming the galvanic circuit. Thus,- a single pair 

Illustrate the natures of the two electricities by the flow of water. State 
the proposition section 192. How may the tension of galvanic electricity be in- 
creased, and made to approximate in its effects to machine electricity ? What 
is said of the quantity of electricity set in motion by a single pair of plates '.' 



GALVANIC ELECTRICITY. 221 

of plates, however large, can never produce electricity of suffi- 
cient intensity to decompose water, or give the slightest shock 
to the animal system ; while, if the same be divided so as to 
form a number of small plates with an equal surface, and these 
be arranged as in Fig. 181, electricity may be excited of sufficient 
intensity to produce sparks, and give vigorous shocks, similar 
to those received from the Leyden Jar : indeed, a galvanic bat- 
tery, composed of a sufficiently extended series, may be made 
to produce nearly the same results as the common electric ma- 
chuie, giving violent shocks, emitting sparks, charging jars, 
etc. 

The increase of the intensity of the electric fluid with the 
increase of the galvanic series is due to the resistance which it 
meets in its passage from one series of metals to another 
through the liquid, each series imparting to it an additional 
momentum or intensity, which becomes greater in proportion to 
the number of the paii'S of metals by which it is urged for- 
ward. 

193. The heating and magjietic effects of galvanic elec- 
tricity depend on its quantity rather than its intensity ; and 
this varies with the amount of chemical action or the extent 
of the metallic surface on u'Jiich this action is exerted. — Thus, 
for extensive heating and deflagration, or for powerful mag- 
netic results, the size of the plates should be regarded. Batteries 
constructed with reference to this, are termed deflagrators. from 
the energy with which metals and other combustible bodies de- 
flagrate or burn, when made to form a portion of the galvanic 
circuit. For heating, and magnetic efiects alone, a single pair 
of metallic plates only is required ; but certain experiments, as 
we shall have occasion to show, depending for their success on 
both quantity and intensity, require that both the size and 
number of the plates forming the series be regarded. 

What is said of the results from dividing such a pair of plates, so as to form 
from them a series of smaller plates? To what is the increase of the intensify 
of the electric fluid in such case due ? . 

19* 



222 



GALVANIC BATTERY. 



194. Galvanic Batteries. — These are constructed of various 
forms and sizes, according to the purposes for which they 
are intended. The general theory of their operation has been 
ah'cady given (*§> 188) ; it only remains to describe a few of the 
more common forms, and give some general directions in regard 
to the manner of their use. 

The simplest form of battery now employed, is that devised 
by Mr. Smee. This is shown in Fig. 189, where a plate of 
platinum or copper, C, forms the electro-negative metal, and 
plates of amalgamated zinc, Z Z, the electro-positive metal. This 
plantinum is usually suspended between two plates of amalga- 
mated zinc,^ from a wooden frame resting on the top of the glass 
receiver. The wires or poles for directing the current of elec- 
tricity, in this, as in the subsequent forms of the battery, are 
connected with the zinc and copper or platinum plates, by means 
of small screw-cups, so as to be easily removed when desired. 
Sulphuric acid, diluted with ten or twelve parts of water, is 
the liquid employed for exciting 
galvanic action. This form of 
battery is extensively used where 
uniform and long continued ac- 
tion is required, as in processes 
of electrotyping yet to be de- 
scribed. 

195. The Sulphate of Cop- 
per Battery^ of which Fig. 182 
presents a sectional view, consists of two cylinders of copper, C C, 
tightly soldered to a copper bottom. Midway between these. 



Fig. 182. 



z 



^p 



* The zinc plates, employed in all batteries where dilute acid is used, should 
be amalgamated with mercury. These amalgamated plates may be prepared 
by pouring upon mercury, in a saucer, some dilute sulphuric acid, and then 
brushing the liquid and mercury over the surface of the zinc, until the whole 
is covered with a bright coat of mercury. 



On what do the heating and magnetic effects of the Galvanic Battery de- 
pend, and how do these vary? Describe the Sulphate of Copper Battery. 



GALVANIC BATTERY. 



223 



bj means of three wood or ivory supports resting on the outer 
cylinder, is suspended a thick cylinder of zinc, Z, so as to re- 
main insulated from the copper surfaces. Two screw-cups, P and 
N, for holding the connecting wires, are attached, one to the 
outer copper, and the other to the zinc. The liquid used is a 
solution of sulphate of copper (blue vitriol) in water.* This 
is poured into the space between the copper cylinders, and acts 
on both surfaces of the zinc, f 
evolving a current of electricity, 
which passes through the liquid to 
the copper, and so out through the 
positive wire, as already explained. 
This battery may be advantageously 
employed for propelling the various 
forms of electro-magnetic engines, 
described in the following sections. 
Fig. 183 presents a perspective 
view of the Sulphate of Copper Bat- 
tery. 




* About two ounces of this salt to the pint is a convenient proportion ; as 
cold water dissolves this slowly, the process may be hastened, when necessary, 
l.y warming the liquid. Sulphate of copper is composed of sulphuric acid 
combined with oxide of copper. 

t The action of this battery is as follows : The oxygen of the water forms 
with the zinc an oxide of that metaL With this the sulphuric acid of the 
salt combines to form a sulphate of zinc, leaving the oxide of copper, which 
is deposited on the surface of the zinc, or falls to the bottom of the liquid, as 
a fine black powder. The hydrogen, the liberated element of the water, in- 
stead of escaping into the air, unites for the most part with the oxygen of the 
deposited oxide of copper, and again forms water, while the pure metal ad- 
heres to the surface of the copper cylinders as a light red powder. 

The zinc cylinder should be removed from the solution, and suffered to air, 
as often as once in fifteen or twenty minutes. When the zinc surface has be- 
come too much coated with the copper oxide, it greatly enfeebles the action of 
the battery, and should be cleaned with a wet sponge, or fine wire card pre- 
pared for the purpose. When the solution has become too much saturated 
with the sulphate of zinc, it should be thrown away, and a fresh supply of 
liquid, prepared as above, should be introduced. 



224 



GROVE S BATTERY. 



196. The Self -Protecting Sulphate of Copper Battery is a 
form of battery, devised by Professor Daniell, for obviating the 
objections to the use of that last described. A cylinder of zinc 
is inclosed in an ox-gullet, or, which is better, a porous earthen- 
ware cup, which separates it from the outer or copper surface 
of a copper receiver. The space within the cup around the 
zinc cylinder is filled with a solution of sulphate of soda 
(Glauber's salts) ; while that without and next the copper is oc- 
cupied by a solution of sulphate of copper (blue vitriol), as in 
the previous section. By this arrangement, whereby two differ- 
ent exciting liquids are employed, such an interchange of elements 
takes place through the interposing earthen, as leaves the surfaces 
of the two metals comparatively clean and free from an oxide de- 
posit, and thus renders the action much longer and more uniform. 

197. A cheap, simple, yet efScient form of Compound Bat- 
tery may be made, by placing plates of zinc and copper in a 
series of glass vessels, and then connecting each set, as shown 
in Fig. 181. The exciting liquid for filling the glasses may be 
sulphuric acid diluted with from ten to fifteen parts of water. 
This series may be extended to any number, forming a battery 
of proportionate intensity and power. 

198. Grovels Battery. — This is an exceedingly efficient form 

of the Galvanic Battery, 
and is now quite exten- 
sively employed for tele- 
graphing and other pur- 
poses requiring quantity 
as well as intensity of 
galvanic action. The con- 
struction and arrange- 
ment of the various parts 

may be learned from 
Fig. 184. Into a plain glass tumbler of about one pint capacity, 

How may a cheap yet efficient form of Compound Battery be made ? What 
is said of Grove's Battery ? How constructed ? Describe the manner of usin^-. 




TROUGH BATTERY. 225 

is placed a thick cylinder of amalgamated zinc, standing on 
short legs, and divided bj a longitudinal opening on one side, 
to allow a free circulation of the liquid. Within this cyl- 
inder is an unglazed porcelain cell, in which is suspended a 
strip of platinum soldered to the end of a zinc arm projecting 
from the adjoining zinc cylinder. When a series of these are 
arranged, as in the cut, to form a compound battery, the terminal 
strip, P, with its screw-cup, is supported by the first zinc cylin- 
der, Z. but is insulated from it by a piece of ivory. Strong nitric 
acid is used in the porcelain cell in contact wdth the platinum, 
and sulphuric acid, diluted with ten or twelve parts of water, 
in the outer vessel containing the zinc* 

199. The Trough ^^z'^^er?/.— This battery, Fig. 185, as 
originally constructed by Mr. Cruik- 
shank, consisted of plates of copper and 
zinc united together in pairs by sol- 
dering at one point only, so as to be 
low^ered into the separate cells of a 

J ^tvKKrvr^^^K^^ ^00^ t^^^S^ 5 t^es^ P^^^^s ^^^®^^® ^*- 
I tached to a strip of wood, and so ar- 
ranged that when lowered into the 
cells of the trough beneath containing 
the exciting liquid, each pair should 
enclose a partition between them. The principle of this arrange- 
ment and action is the same as shown by Fig. 181 ; it possesses, 

* Owing to the strength of the acids used, and the widely different oxidizable 
qualities of the two metals employed, the chemical action of this battery is 
great ; and the quantity and intensity of the electricity evolved from a given 
surface exceeds that from any other form of battery now used. 

This form of battery is, however, objectionable for common purposes, 
owing to the risk attending the use of such strong acids by inexperienced ma- 
nipulators, and the corrosive nature of the nitrous acid fumes given off during 
its action ; these being not only injurious to health, but acting readily on the 
polished metals of a close room. 

How are the plates of the Trough Battery arranged ? 




22G 



TROUGH BATTERY. 



lioweverj a decided advantage over that, in the fact that the 
whole series maj be lifted at once from the cells, when desired, 
and further action thus checked. 

Fig. 186 shows a modern and far superior arrangement of the 
Trough Battery. In this form the zinc plates are inclosed in 
copper cases, open at the top and bottom, thus doubling the sur- 
face as compared with the last.* 



Fig. 186. 




The whole series is also confined in a wood case, B, which 
may be raised or lowered into the trough, A, containing the acid, 
by means of the windlass, C. E E are small hand- vices con- 
nected with the poles for holding wires, bits of charcoal, etc., 
for deflagration. The exciting liquid is sulphuric acid diluted 
with about thirty or forty parts of water ; where extraordinary 
action is required the proportion of this acid may be increased. 

The greatest power of this battery is attained soon after the 

* The <arrangement of these plates is shown in cut at the left. Each plate 
is held firm in its coppei' by bits of grooved wood arched to fit the copper case 
and placed at its top and bottom. Between the copper cases are fixed thin 
strips of veneer. These, with the bits of wood just referred 
to, are boiled in a mixture of oil and resin, to render them 
impervious to water, and thus improve their insulation. The 
connection between the series is formed as shoAvn at 1 1, a strip 
of copper leading from the copper case being soldered to a pro- 
jection on the zinc of the next set. The arrangement, H H, 
etc. , is designed to show the manner of converting this battery into a calorimeter, 
by connecting all the copper and zinc plates so as to form one continued surface. 




ELECTRIC AFFINITY. 227 

immersion of the plates ;* advantage should be taken of this 
fact, and these not suffered to remain too long in the acid, but 
often raised and allowed to air. 

For purposes of decomposition and deflagration, the Trough 
Battery possesses superior advantages, combining both conven- 
ience and efficiency. The expense and skill requisite in their 
construction have caused a quite general substitution of Grove's 
Battery. Fig. 186 shows a battery of fifty pairs, which, for the 
actual space occupied, exceeds in power all other forms. 

200. The important relations of the galvanic battery to re- 
searches in this department of electricity, have led us to enter 
thus at length into a description of the principal forms ; we noAV 
pass to consider further some of the principles which it unfolds. 

201. In every chemical combination the force by which the 
elem>ents are held together is regarded as an electrical force. 
— We have already shown, § 151, that bodies in opposite elec- 
trical states, are attracted to each other. Thus, if two elements 
of a chemical compound, charged with opposite electricities, are 
brought sufficiently near, these will be attracted and held 
together by a force proportioned to the difference of their elec- 
trical states. Thus, the difference in the electrical states of 
oxygen and hydrogen (the elements of water) is comparatively 
slight ; hence the feeble affinity existing between these, and the 
ease with which they are separated when united to form water. 
Oxygen and potassium (the elements of potash), on the other 
hand, differ widely in these respects ; the former being at the 
extreme of the electro-negative, and the latter at the extreme 

* This battery should be wet up with an extremely weak solution some time 
previous to use, and additional strength given to this when wanted for action. 
All contact of the plates, or communication between these, by means of straws, 
etc., should be carefully avoided, as by such neglect the power of the battery 
is oftentimes wholly defeated. 

In case of decomposition or deflagration, what is said of the Trough Battery ? 
By what force is it supposed that the elements of a chemical compound are 
held together ? With what proportional force are the elements of such united ? 
Illustrate this in the case of oxygen gas and potassium. 



228 



ELECTRIC DECOMPOSITION. 



of the elcetro-positive elements. As a consequence, these are 

drawn together, and united by a force superior to that of any 

of the binary oxides, and require for their separation agencies 

of the most intense decomposing energy. Thus, all chemical 

affinity has been regarded by Faraday and other distinguished 

electricians as simply a modification of electric attraction. 

202. Experiment. — Let t\Y0 glass tubes be filled with 

water, rendered slifi!;htly salt 
Fig. 187. . , , , . ° i . 

or acid,* and mverted m a 

cell of the same liquid, over 
two wires, terminated by 
small strips of platinum, as 
shown by Fig. 187. If, 
now, these AYires be connected 
with the poles of the bat- 
teries. Fig. 184 or 186,t 
small bubbles will be seen to 
escape rapidly from the plati- 
num strips, rising and dis- 
placing the water of the 
tubes ; the tube h connect- 
ed with the negative pole 
of the battery, containing 
at any time about twice the , 
volume of gas found in the 
tube, 0. connected with the positive pole of the same. Place 
corks in these, and invert. If a lighted taper be now applied 
to the mouth of the tube. A, the gas will burn with a dull flame, 
showing it to be hydrogen ; if the same taper be extinguished, 

* These increase the power of the liquid to conduct electricity, and thereby 
facilitate its decomposition. 

t A large-sized Sulphate of Copper Battery, Fig. 183, may be advantageously 
employed, by interposing the Vibrating Shocker, Fig. 22G, between it and the 
Decomposing Cell. 




How may the elements of which water is composed be separated ? 



ELECTRIC DECOMPOSITION. 229 

except a lingering spark, and placed in the tube, o, it will be 
instantly relighted, proving the presence of oxygen. 

The theory of this decomposition is as follows : By means of 
galvanic action the electrical affinity of the two elements for 
each other is overcome ; the hydrogen, the positive element of 
water, being drawn to the negative pole of the battery, while the 
oxygen, the negative element, passes to the positive pole. By 
this means, moreover, the relative 'projportions of the elements 
in water and all other chemical compounds are determined, and 
found to be definite and unvarying. Thus, in the case of 
water, as just shown, the proportions are two parts by volume 
of hydrogen, combined with one part of oxygen.* 

Experiment a. — In place of the double tubes, as in the 
last experiment, fill, and insert the single tube, h o, causing 
both points to enter its mouth. Connect with the battery ; the 
tube will soon be filled with a highly explosive mixture of 
oxygen and hydrogen gases, as may be shown by inverting and 
applying a lighted taper. By such an explosion the gases will 
unite, and again form water. 

In these experiments the volume of gas liberated is in exact 
proportion to the quantity of electricity passing through the 
liquid. This decomposing cell is hence termed a Volta-scope^ 
or measure of electricity, f 

203. In this decomposition of water by galvanic action, as 
in that of other chemical compounds, the process by which the 
elements are simultaneously evolved at the poles of the battery 
is supposed to be briefly as follows : 

* The simple or elementary substances, of which matter in its almost infinite 
variety of foi-ms is composed, are supposed to be only about sixty ; forty-eight 
of these are metallic and twelve non-metallic substances. 

t In the decomposition of liquids, regard must be paid to the quantity of 
these. A small battery will act only on a proportionably small amount of 
liquid. Ignorance of this fact often occasions failures in the use of the De- 
composing Cell, Fig. 187. 

What proportions do oxygen and hydrogen unite to form water ? Theory of 
the decomposition of water by the galvanic current? 

20 



230 ELECTRIC DECOMPOSITION. 

Each atom of water, for instance, is composed of two ele- 
ments, oxjgen and hydrogen. Now, suppose a series of these 
atoms to lie between the battery poles ; that atom at the extrem- 
ity of the series next the positive pole, for instance, will bo 
decomposed, its oxygen being drawn to, and evolved at, this 
pole, while its hydrogen^ being repelled from this, and attracted 
toward the opposite negative pole, will force along the hydro- 
gen of the next adjoining atom, and unite with its oxygen, 
while the hydrogen of the next atom in the series will, in its 
turn, be displaced ; and so a series of decompositions and recom- 
positions will take place through the whole line, until coming 
to the last, next the negative pole ; the hydrogen driven off 
from this, having no element with which to combine, is evolved 
in the form of a minute bubble of gas, as shown by Experiment 
^202. Thus, no actual transfer^ but simply an interchange of 
the elements of the compound, is effected, whereby those at the 
extremities of the series next the galvanic poles are liberated 
and appear. By a similar process of interchange the decom- 
position of the more complex alkaline compounds is produced. 

204. Experiment. — Let the poles of the battery be ter- 
minated by small strips of platinum, placed in a bent tube 
fixed in a stand, as shown by Fig. 188. Pour into this tube 
a solution of sulphate of soda, to 
which has been added a sufficient 
infusion of red cabbage to give to 
it a blue color. Upon the passage 
of a current of electricity, the 
blue color will soon be changed to 
red in the arm containing the 
positive pole, and to green in the 
one containing the negative pole; 
showing in the former the presence 

State the supposed process of interchange of elements in the decomposition 
of each atom of water. What change will be effected in an infusion of red 
cabbage, through which a galvanic current is made to flow as shown in Fig. 188 ? 




ELECTRO-METALLURGY. 231 

of sulphuric acid — one of the compound elements of the salt 
— and, in the latter, that of soda, its other compound element. 
Now, reverse the current, and the original color will first be 
restored in each arm of the tube, and then the opposite change 
will occur. 

Experiment a. — Fill the tube. Fig. 188, with a solution 
of iodide of potassium, to which has been added a little starch, 
and send ttyt'ough a galvanic current. The iodine will be freed 
fi-om its combination, and appear at the positive pole, giving 
Y^ ith the starch an intensely blue color to the liquid in the cell 
containing that pole. 

205. Electro-Metallurgy . — When metallic salts, as sulphate 
of copper, are dissolved in water, and a current of electricity from 
a galvanic battery passed through the solution, such salts are 
decomposed (see § 195, note) ; the oxygen or electro-negative 
element of the base * going to the positive pole, while the 
metal is deposited on the wire, or other metallic surface at the 
negative pole. If, in this case, proper skill be exercised in 
regulating the electric current, and the consistency of the saline 
solution, the deposit will take place with remarkable evenness 
and regularity, forming what is termed the reguline deposit. 

Experiment. — Connect by soldering, or otherwise, to the 
negative wire (that leading from the zinc plates), of a Smees 
Battery, a medal, coin, or other metallic object. Smear the 

* The base, is the principal ingredient in a chemical compound, or that with 
which acids, etc., combine. In this case, oxide of copper (copper rust) forms 
the base, with which sulphuric acid combines to form the salt, sulphate of 
copper. 

What change will be effected in a solution of iodide of potassium ? What is 
the effect of passing a galvanic current through water in which is dissolved a 
metallic salt, as sulphate of copper ? In this case, what is said of the deposit 
of the metal ? If proper skill be exercised in regulating the electric current 
and consistency of the solution, how will the metal be deposited on a metallic 
surface ? State the manner of preparing and arranging the objects on which 
a metallic deposit is to be formed. 



232 



ELECTRO-METALLURGY. 



back and edges with a coating of wax or varnish, to protect 
these parts against the eifects of galvanic action. Attach to the 
end of the other positive wire a small piece of sheet-copper, and 
then immerse this with the medals, etc., in a solution of sulphate 
of copper, in a glass tumbler, with the unprotected surface next to 
and near the copper. Fig. 189 shows a convenient arrange- 

Fig. 189. 




ment, where three separate medals are being copied by the 
action of the same galvanic current. A bright film of pure 
copper will soon form on the coin or medal ; and, if the action 
be continued for a day or more, the metal will acquire con- 
siderable thickness, and may be removed, when it will be found 
to present a smooth and perfect transcript of that portion of the 
surface on which it has been deposited. 

Experiment a. — Form a smooth and perfect mould by im- 
pressing on wax, stearine, or fusible metal when in a plastic state, 
a medal, wood-engraving, or other object. When hard, cover 
this mould with a smooth and delicate coating of plumbago ; * 
connect this with the negative wire of the battery, and immerse 
ia a solution of sulphate of copper,f as in the previous experi- 

* The copper wire should be placed carefully around the edge of the mould, 
so that the deposit, which begins at the wire, may be on all sides alike. The 
plumbago may be put on with acamel's-hair brush. This serves as a conduct- 
ing surfjice. 

t Pieces of the sulphate of copper should be from time to time placed in the 
tumbler, as the solution becomes weak and palo. 



HEAT FROM ELECTRICITY. 233 

ment. A smooth and firm deposit of copper will, in the course 
of a day or two, be made on the portion covered with the plum- 
bago, and, when removed, will be found to contain all the nice 
irregularities of the original. In this manner duplicates of 
wood-cuts, type, etc., may be obtained, from which impressions 
can be taken fully equal to those of the originals. 

206. By a somewhat similar process, gilding with the precious 
metals is effected : these being deposited from their solutions on 
metallic surfaces by the same galvanic action.* 

Since its discovery by Professor Jacobi, in 1837, Electro- 
Metallurgy has been carried to a wonderful degree of perfection, 
and now holds an important rank among the useful arts. For 
further information upon this interesting subject the reader is 
referred to works especially devoted to this art.f 

207. The heat evolved by an electric current is in propor- 
tion to the resistance offered to its passage through a body. — 
In respect to their powers of conduction, metals differ widely ; 
thus, while a silver or copper wire will transmit the electric cur- 
rent from a battery freely and without heat, a platinum or steel 
wire of equal size and length may afford so much resistance as 
to heat and melt or even ignite the wire. 

Experiment. — Place between the poles of a Grove's or a 
Trough Battery, of medium size, one or two feet of number thirty 
steel wire. Upon the passage of the galvanic current, this 

* An improTed manner of electro-gilding has been recently devised by a 
German, and adopted by a portion of the artists of this city. Instead of the 
battery heretofore employed, the electric cui'rent is produced by a magneto- 
electric machine or dry action. By this arrangement the deposit is formed 
Avith far greater rapidity than by that of the common battery. 

t Davis' Manual of Magnetism and Walker's Electrotype Manipulations 
may be profitably consulted. 



Tlow may metallic copies of medals, ivood-engravings, etc., be formed? 
v. iiat is said of the heat evolved by the passage of an electric current through 
a body? Do all metals conduct electricity alike? What illustration given of 
this ? State the experiment for igniting steel tvire. 

20* 



234 



EXPLOSION BY ELECTRICITY. 



will become intensely heated, and ignite with brilliant scintilla- 
tions.^ Platinum wire of a larger size may be burned in the 
same manner. 

Experiment a. — Inclose in a small and thin glass tube, 
about one foot in length, a spiral of small steel or platinum 
wire, coiled as close as may be without a contact of the spirals. 
Lot the ends project through corks, and connect with the poles 
of the battery, as in the last experiment. The electric current 
will soon heat the wire and tube, so that if the latter rest in a 
small quantity of water, this may be raised even to a boiling 
heat. 

Experiment h. — Place in the Powder-Cup^ Fig. 190, some 

fine powder, and con- 
nect its wires with the 
poles of a Sulphate 
of Copper Battery ; 
the passage of even a 
feeble current across 
the fine platinum 
wire, which joins their 
ends within the cup, 
will so heat this wire as to explode the powder, f 

The Galvanic Pistol^ Fig. 
191, is provided with an ar- 
rangement similar to that of 
the Powder- Cup in the pre- 
vious experiment. Two in- 
sulated wires or poles pass 
through the screw-plug, o, 




Fig. 191. 




* The Universal Discharger, Fig. 1G5, Mechanical Electricity, may be con- 
veniently used for holding these wires. 

t By means of a platina wire passing through the powder of the priming- 
hole, experiment § 166 may be performed by the Galvanic Battery instead of 
the Leyden Jar. 



How is the powder ignited in the Powder-Cup? TTow may explosive gases 
be tired by the galvanic current ? 



GALVANIC ILLUMINATION. 235 

upon, the under side of the pistol, and are joined at their extremi- 
ties bj a platinum wire. 

Experiment c. — Fill this pistol with an explosive mixture, 
as directed in § 143, and, with the cork tightly inserted, connect 
the wires with a Sulphate of Copper Battery, when the plati- 
num wire will become of a glowing heat,* and explode the 
gases, forcing out the cork with a loud report. 

Experiment d. — Hold in the vices or pincers attached to 
the poles of a Trough or Grove's Battery two small pointed cyl- 
inders of boxwood charcoal. With the battery at its highest 
point of action, bring these charcoal points together, when a 
spark will pass, and the points become ignited. These may now 
be separated to a greater or less distance, according to the power 
of the battery, and an arch of light of the most intense bril- 
liancy will be formed between the points. The ignition of the 
charcoal in this experiment is independent of the aid from oxy- 
gen gas, as in ordinary combustion, since the same light may be 
produced equally well in a vacuum. The cause, however, ap- 
pears to be due to the transfer of the particles of charcoal in a 
state of ignition from the positive to the negative pole. Both 
the quantity and intensity of the galvanic current are to be re- 
garded in this experiment, and hence the larger the plates, and 
the more numerous the series, the greater will be the brilliancy 
of the result. 

* In all galvanic experiments, "where the connections are in part of fine plati- 
num or steel wire, regard should be had to the power of the battery, lest the 
quantity of the galvanic current be so great as to melt or ignite these wires. 
Such experiments are generally best performed by the Sulphate of Copper 
Battery. 

What is said of the light from charcoal points ignited by the Galvanic 
B:\ttery ? In the case of the intense ignition of these charcoal points by a cur- 
rent of galvanism, is the presence of atmospheric air or oxygen gas necessary ? 



236 



PHYSIOLOGICAL EFFECTS OF GALVANISM. 



PHYSIOLOGICAL EFFECTS OF GALVANISM. 

208. Whenever any portion of the body of an animal, liv- 
ing or recently killed, is made to form, a jmrt of the cjalvanic 
circuit, a violent contraction of the muscles of that portion 
takes place. 

The nerves of certain animals form one of the most delicate 
tests known of the passage of an electric current. As we have 
jilready remarked, §158, it was the twitching of the legs of frogs 
occasioned by the contact of two different metals, that first sug- 
gested to Galvini those experiments which led to the discovery 
of the wonderful science which bears his name. This singular 
phenomenon may be shown by the following 

Experim^ent. — Let two pieces 
of metal, one of silver, S, and 
the other of zinc, Z, be fixed 
in a small block of dry wood, 
as seen in Fig. 192, and let the 
former be so curved as to touch 
the latter, upon a slight pressure 
of the finger, as shown by the 
figure. Remove the hind legs 
of a large frog, recently killed, 
from the body, so that they shall 
remain joined together, and 
place these between the metals 
so that one metal shall touch the 
large nerve proceeding from the 
legs, and then the other the mus- 
cle where the skin has been strip- 
ped off. Upon a contact of the 
metals above the block, a galvan- 
ic current will pass, and the legs 




State the effects of galvanism on the animal system. What is said in regard 
to the nerves of certain animals ? How may the effects of galvanism upon the 
le"s of a fropj be shown ? 



PHYSIOLOGICAL EFFECTS OF GALVANISJI. 237 

'wliich hang, as indicated by the dotted lines, will instantly un- 
dergo a wonderful contraction, and be drawn up as seen in the 
cut.* 

Experiment a. — Place on a moistened plate of zinc a 
piece of silver or platinum, and on the latter put a leech or 
earth-worm. As often as the animal attempts to leave the silver, 
it will receive a violent shock, as will appear from its sudden 
withdrawal and return to this again. 

Experiment b. — Place a piece of zinc above and one of 
silver below the tongue, when well moistened by saliva. Make 
a contact of these metals thus arranged, and a slight twinge, 
accompanied by a disagreeable metallic taste, will be the result. 
This is due to the oxidation of the zinc and the passage of a 
galvanic current. 

209. By a powerful galvanic battery, convulsions, similar to 
those of the legs of the frog, may be produced on the bodies 
of larger animals and men, soon after death, such as to induce a 
belief that the animal has been restored to life, and is enduring 
the most cruel sufferings. Thus, if the two wires from the 
poles of a large battery be inserted in the ears of an ox or 
sheep, when the head has been removed from the body of the 
animal recently killed, the most surprising convulsions will 
result as often as the galvanic current is made to pass. 
The eyes will be made to open and shut, and roll in their sock- 
ets, as though again endued with vision ; the nostrils will 
vibrate, as in the act of smelling, and the jaws imitate all the 
movements of mastication. The experiments of Dr. Ure, upon 
the body of a muscular, athletic man, who had been hung for 
murder, are truly wonderful. When the galvanic current from 

* Davis' Manual. 

In Experiment a, "why does the leech or earth-worm draw back as it touches 
the zinc ? W' hat effects may be produced by the action of a powerful battery 
on the bodies of animals recently killed ? Effects in traversing the head of an 
ox or sheep ? Experiments of Dr. Ure upon the body of a murderer? 



238 PHYSIOLOGICAL EFFECTS OF GALVANISxM. 

a very powerful battery Avas passed through portions of the 
body, all the motions of life were exhibited ; laborious breath- 
ing commenced, tlie diaphragm rose and fell, the muscles of 
the countenance were simultaneously thrown into fearful action ; 
rage, horror, despair, and ghastly smiles, united their hideous 
expression in the face of the lifeless murderer, while the arms 
and fingers exhibited convulsive movements, and seemed to 
point to the different spectators, some of whom believed life had 
returned. =^ 

* Noad's Electricity. 



THERMO-ELECTRICITY. 239 



THERMO-ELECTRICITY. 



210. Professor Seebeck, of Berlin, in 1822, discovered that 
by joining two different metals, possessed of different conduct- 
nig powers of heat, and heating these at the point of junction, 
a current of electricity would be caused to flow from the colder 
to the hotter metal. Electricity thus developed by the agency 
of heat is called Themio-'Ehctricitj. 

211. Experiment. — Let two strips, one of German silver, 
G, and the othei of brass, B, Fig. 193, be brazed together, or 



Fig. 193. 

G 



merely touch each other at one of their extremities, and let 
them be so arranged as to form an acute angle, and have small 
copper wires attached to their free ends, as seen in the figure. 
Upon applying the heat of a spirit-lamp at the point of junc- 
tion, a very perceptible current of electricity will flow from the 
German silver through the brass, whenever the copper wires 
are so connected as to form a complete circuit. The quan- 
tity of the electricity thus set free may be indicated by the 
deflection of the needle of the Galvanometer, Eig. 198, when 
the ends of the wires are made to connect with this instru- 
ment. 

212. Two plates of German silver and antimony, heated, 
at the point of junction, by immersion in oil raised nearly to 
the melting-point of the latter metal, will set free electricity 
in greater quantity than any other combination of metals. 
Different degrees of temperature in the same metal will occa- 

Define Thermo-Electricity. How may this be produced ? How may the 
quantity of electricity set free by this process be indicated ? What combination 
of metals, and how heated, to produce the greatest flow of thermo-electricity ? 



240 THERMO-ELECTRICITY. 

sion an electric flow from the colder to the warmer portion, 
■which may be made perceptible, provided the metal be a poor 
conductor of heat, as in the case of platinum. 

213. A thermo-electric battery of considerable power may 
be constructed, by soldering together alternate plates of German 
silver and brass or antimony, at such an angle as to allow the 
interposition of sheets of pasteboard, to prevent a contact of 
the metals, except at their ends or junctions. Such a battery 
is shown in Fig. 194. By the application of heat to such a 

Fig. 194. 




series, an electric floAV will take place proportioned to the num- 
ber of the series and the degree of heat. Such an arrange- 
ment, formed by small bars of bismuth and antimony, becomes 
far more sensitive to the effects of heat than either the mer- 
curial or air thermometer ; so that even the radiant heat from 
the hand brought near one end of these bars, will excite 
sufficient electricity to deflect the needle of a delicate galva- 
nometer several degrees. 

The thermo-electric current thus excited between two dif- 
ferent metals is referred to the difference in their conducting . 
power for heat, and to the different orders of crystallization 
to which their particles belong ; the laws of crystallization 
being supposed to result from the electrical character of the 
particles. This has not, however, been fully investigated, and 
many points are involved in great obscurity.* 

* Davis' Manual Magnetism. 

Effect of diflFerent degrees of temperature in the same metal ? How may a 
thermo-electric battery be formed? To what causes is the electric current, 
thus formed, referred ? 



ANIMAL ELECTRICITY. 241 



ANIMAL ELECTRICITY. 

214. This term is applied to that form of electricity pro- 
duced by certain fishes, as a means of defence, or for the 
capture of their prey. Among these the Gymnotus, the Tor- 
pedo,* and the Silurus, are the most remarkable examples. 
These are each provided with a special set of organs for 
setting free electricity when desired for effecting a shock ; and, 
although differing slightly in the form of the arrangement, yet, in 
the principle of their action, these organs are the same in each. 

In the Gymnotus the electric organs consist of long mem- 
braneous structures, extending on each side of the spine from 
the head to the tail, and are divided by numerous septa into 
little cells filled with a gelatinous fluid for exciting electric 
action ; thus bearing a striking analogy to the arrangement 
of the Compound Galvanic Battery. These organs are copiously 
supplied with nerves branching off from the spine, and appear 
to act through the nervous agency, subject to the control of 
the will of the animal ; accordingly, if these nerves be severed, 
all power of the will over this galvanic arrangement ceases, the 
same as over the muscles of a limb when the nerves supply- 
ing them are cut. 

215. The Gymnotus, or electric eel, is found in the waters 
of South America, and bears a strong resemblance to the com- 
mon eel of this country ; varying in length from two to five 

* The Torpedo is a flat fish, found along the shores of the Atlantic, varying 
in length from one and one half to four feet. Its electric organs are two 
in number, and lie one on each side of the head or gills. The electrified sur- 
face of these is very great, equivalent in some instances to a thousand feet of 
coated glass. When taken with a harpoon it sometimes transmits through this 
a severe shock to its captor. 

Define Animal Electricity. What animals evolve this in the greatest 
degree? With what are these animals provided? The form of the electric 
organs of the Gymnotus ? With what are these organs provided ? Eft'ect of 
severing these nerves ? Where is the Gymnotus found ? 

21 



>12 



THE GYMNOTUS. 



feet. Fig. 195 presents vievrs of this animal in two positions. 
The lower is a lateral view, and the upper a view from above. 
When disturbed bj the entry of any animal into their watery 




realms, as a horse, for instance, these fishes glide along near 
or in contact with the body of the animal, and transmit a 
powerful shock, which, if repeated, may succeed in prostrating 
their victim. Humboldt relates sccino; a heid of horses driven 



Case of a herd of liorscs doscilbcl V>v HnmboMt "' 



THEORY OF MUSCULAR ACTION. 243 

into a pool, where were several of these gjmnoti, and attacked 
bj the latter with such vigorous shocks, as soon to be over- 
powered and made to shik powerless in the water; while, at 
the same time, the fish appeared to become exhausted as from 
a severe muscular effort. 

A course of ingenious experiments with a Gjmnotus was 
made some years since, by Dr. Faraday, whereby he proved the 
complete identity of this animal electricity w^ith that evolved 
by artificial means; producing not only shocks, but decompos- 
ing and magnetic effects, the same as by the ordinary battery. 
In these discharges, the electric fluid passed from the head 
towards the tail ; the former being the positive, and the latter 
the negative extremity of the animal battery, 

216. Muscular action and animal electricity appear to be 
results of the same nervous or vital force, both alike requiring 
an expenditure of nervous energy for their production ; so that 
whichever of these effects is produced, a proportional waste of 
the animal system results. 

Leibig has accordingly suggested a theory of muscular action, 
which supposes that the contractile force of the muscles is due 
to a principle set free by the oxidizement of the animal tissues 
by the blood, in the same way that electricity is set free by the 
chemical action of acids on zinc. He supposes that, when 
muscular contraction takes place, the nerves supplying the part 
withdraw their vital protection, and oxidation under the chem- 
ical laws, and the consequent development of force, result. 
A further application of this theory would extend it to the 
electrical organs of fishes, where, by oxidizement of tissues suit- 
ably arranged, electricity itself, instead of muscular force, 
would be eliminated whenever the protecting agency of the 
nervous system was withdrawn.* 

* Davis' Manual. 



What did Dr. Faraday's experiments upon the Gymnotus prove ? What is 

said in regard to muscular action and animal electricity ? Theory of Leibig. 



244 ANIMAL ELECTRICITY. 

217. During certain diseases of the nervous system the 
human subject has been known to emit electric shocks. In 
such cases the nervous energy, as in the Gymnotus, seems 
directed to the production of electrical instead of muscular 
force, as in ordinary states of the system. Free electricity 
is found to be an invariable result of chemical change ; con- 
sequently, the animal body, which is subject to constant and 
extensive changes of this nature, is found to set free large 
quantities of electricity. Now, if in certain abnormal states 
this be subject to the control of the will, acting through the 
nervous agency, as we have supposed, may we not reasonably 
infer that animal electricity exerts in some way an agency for 
producing the mysterious phenomena exhibited in the move- 
ments of inanimate matter in certain relations to the human 
subject ? 

Matteucci found, by experiment, that currents of electricity 
were constantly passing between different parts of the living 
body. Thus, by making a metallic communication between the 
liver and stomach of a live rabbit, he found a powerful galvanic 
current setting from the one organ towards the other. Hence, 
some distinguished physiologists have been led to ascribe to 
electricity an important agency in digestion, and the secretions 
of the animal body. However this may be, there can be but 
little doubt that electricity plays an important part in the 
economy of the animal system, which science may sooner or 
later unfold more fully to our understanding. 

Case in certain diseases of the nervous system ? Effect of chemical changes 
in the animal system? What did the experiments of Matteucci show ? What 
agency is ascribed to electricity by some distinguished physiologists ? 



MAGXETISM OF ELECTRIC CURRENTS. 



245 



ELECTRO-MAGNETISM. 



218. The influence of electricity in communicating Magnetism 
to bars of iron and steel, and also in destroying or reversing the 
polarity of the magnet, has been known for ages ; but not 
until the discovery by Professor Oersted, of Copenhagen, in 1819. 
was the power of an electric current, in giving direction to a 
magnet, or the peculiar reciprocal force between this and the 
magnet, known. 

219. Electric currents exert a magnetic influence at right 
angles with the direction of their flow. — If the wire con- 
veying a galvanic current be made to pass near a nicely bal- 
anced bar-magnet, lying in the same direction, this will be at 
once deflected, and made to take a position at, or approaching 
to, a right angle with the wire. 

Thus, in Fig. 196, let A B represent such a wire, and S N 
a magnetized bar lying directly 
beneath and in the same direc- 
tion with the wire. Upon the 
passage of an electric current 
the magnet is at once deflected, 
and tends to a position, s n, at 
right angles with the wire. 
Place the wire as before, but 
beneath the magnet, and allow 
the electric current to flow 
through it in the same direc- 
tion as when placed above; the 
magnet will be at once reversed, 



Fig. 196. 



3 



N< 




=^8 



What is said of former knoTvledge in respect to the influence of the electric 
current in communicating Magnetism ? In what direction do electric currents 
exert their magnetic influence ? State the efiects of such a current when 
passing along a wire near a nicely balanced bar-magnet lying in tlie same 
direction. Effect of so bending the wire as to send the current around the 
magnet ? 

21^ 



246 



GALVANOxAIETER. 



Fig. 197. 




S standing where N, and N where S did previously. If the 
wire be bent, so as to convey the current around the magnet 
above and below in opposite directions, these opposite currents 
will exert upon the magnet forces auxiliary rather than antag- 
onistic to each other. 

220. The Galvanometer^ Fig. 197, designed for measuring 
the quantity of an electric cur- 
rent, will serve to illustrate this 
more clearly. The instrument 
shown in the figure consists of 
a single wire, bent at right 
angles so as to pass above and 
beloAY a nicely balanced magnet, 
N S ; the point where the parts 
of the wire pass each other near 
"^^^^ C, being insulated by winding 

with a little thread. 

Attach the wires of a galvanic battery to the screw-cups, A 
and B, so that electricity shall flow through only the portion of 
the wire upon the upper side of the magnet ; this will be de- 
flected to a certain extent. Let the connection now be made at 
A and C, so as to send the current above and below the magnet 
in opposite directions. A much greater deflection than before 
will take place. 

If, instead of a single turn, as in this case, the wire be 
carried several times around the magnet, the magnetic force 
exerted by a current of electricity traversing this will be in- 
creased in proportion to the number of these turns, within 
certain limits. Galvanometers thus constructed are termed 
'inultijpliers^ since they serve to increase the power of the elec- 
tric current, and indicate its flow even in the smallest quan- 
ities. 



Design of the Galvanometer, Fig. 197 ? Effects of the galvanic flow through 
only the upper side of th^ wire surrounding it? Through the entire wire? 
Effect of the galvanic flow through a wire making several turns about the 
magnet? What are galvanometers thus constructed called, and ■\^hy ? 



ASTATIC NEEDLE. 



247 




Fig. 198 shows a common form of the Galvanometer for 
indicating the direction and quantity 
of the galvanic flow. A wire is coiled 
several times about a magnetic needle, 
with its two ends terminating beneath 
the screw-cups. The needle is made 
to stand in a line with the coil when 
acted upon by the earth's magnetism 
alone. Upon the passage of an electric current, this is deflected 
to a greater or less angle, varying with the quantity of the 
flow ; the amount of these deflections being indicated by a grad- 
uated card pasted on the stand beneath. Thus, by means of 
the Galvanometer properly constructed, currents of electricity, 
by far too feeble to be detected by ordinary means, may be made 
to affect sensibly a magnetic needle. 

221. As the magnetic attraction exerted by the earth affects 
somewhat the sensitiveness of the needle to electric influences, 
Nobili devised the Astatic Needle, for obviating this difficulty. 
This is seen in Fig. 199, and consists of two similar magnetic 
needles, placed one above the other in 
positions the reverse of each other in 
respect to their poles. Thus, its 
directive tendency in respect to the 
earth is neutralized, so as to allow it to 
remain at rest in any position, and so 
rendering the influence of the galvanic 
flow more perceptible. 

222. The poles of a magnet, as has 
been already shown, observe certain in- 
variable positions, in reference to the 
direction of the electric current. In order to impress on the mem- 
ory these positions, Ampere has suggested the following formula : 
Let the jjerson suppose himself lying on the loire vAth his 



rig. 199. 




"What does Fig. 198 represent? Describe the Astatic Needle. State the 
formula of Ampere. 



248 ELECTRO-MAGNETISM. 

face towards the inagnet^ and the electric current flowing 
from his Jtead toward his feet ; the north j)ole of the mag- 
net will always be toward tlte right hand. By bearing in 
mind this simple rule, the direction of the flow of an electric 
current may be always readily determined, upon observing the 
position of the poles of a magnetic needle in reference to it. 

Thus, from what has been said, it will be seen that, unlike all 
other motive powers in nature, electricity exerts its magnetic 
force laterally^ instead of in the line of its direction. Nor 
does the magnetic pole move either directly towards or directly 
fi'om the conducting wire, but tends to revolve around it with- 
out changing its distance. Hence, the force exerted upon the 
magnet must be considered as acting in the direction of a tan- 
gent to the circle in which the magnetic pole would move.* 

In the illustrations Ave have given, the action of the electric 
current has been exerted alike on both poles of the magnet in 
contrary directions, causing these to assume a state of equi- 
librium in a direction transverse to the path of the current or 
wire conveying it. 

223. If the conducting wire of a7i electric current be 
made to pass near a single pole of a Qnagnet free to Qnove^ 
this pole ivill commence a revolution around the wire in a 
direction depending on the course of the current. — Thus, 
if the north pole of a magnet be presented to a vertical 
wire, through which a stream of electricity is descending.^ it 
will, if free to move, revolve about the wire in the direction 
of the hands of a watch. If the current be made to ascend.^ 
the pole will take an opposite direction. With the south pole 
thus situated, the direction of the motion will be reversed. 

These movements of a single magnetic pole about a con- 

* Davis' Manual. 



What is said in regard to the direction in which the electric current exerts its 
ma^^netic force ? Is the direction of this the same as other forces? State tlie 
proposition. Illustrate this. 



ELECTRO-MAGNETISM. 



249 



ducting wire may be illustrated by an arrangement shown in 
Fig. 200. A magnetized bar, S N. bent at right angles in its 
middle, rests on an agate, or other in- 
sulating support, at N. A vertical 
wire is fixed in the axis of motion, to 
which the upper pole, S, is so attached 
by a small loop as to allow the magnet 
to revolve freely about it. This wire, 
connecting with the screw-cup. A, 
has its lower extremity resting in a 
small cup, placed upon the hori- 
zontal portion of the magnet. From 
this cup projects a wire, bent so as to 
terminate in a circular cistern of mer- 
cury, open in its centre to allow a revo- 
lution of the magnet, independent of 
any contact with it. With this mer- 
cury-cistern is connected a second 
screw-cup, B. 

Experiment. — Attach the positive 
pole of a galvanic battery at A, and 
the negative at B ; the galvanic current will flow down the ver- 
tical -wire, near the upper pole, S, of the magnet, and pass off 
at C, through the bent wire and mercury, to B. Thus, as but 
a single pole of the magnet is acted on, it will commence a 
rapid revolution round the conducting wire, in the direction 
and according to principles already stated. 

As action and reaction (§20) are always equal, if the con- 
ditions of the magnet and the conducting wire in this experiment 
be changed, so that the former shall be permanent, and the 
latter free to move, upon transmitting a galvanic current 




What does Fig. 200 illustrate ? Explain this instrument. How will the 
passage of the electric current along the wire, near the upper pole of the 
magnet, affect it ? Result of changing the conditions of the wire and magnet 
in this experiment ? 



250 



ELECTRO-MAGNETISM. 



through the Avire, this will be made to revolve about the 
magnet, the same as the magnet about this in the experiment. 
224. If a conductincj ivire^ free to move^ he submitted to 
the action of both j^oles of a magnet^ it will move forxoard 
in a line hettveen these two poles. 

This may be shown by the arrangement seen in Fig. 201. 

From a small mercury-cup, 
at the end of a horizontal 
rod attached to an upright 
post, is suspended a copper 
wire. W, with its lower 
extremity, when at rest, 
just entering a small basin 
of mercury in the stand. 
The wire hangs so as to 
vibrate freely between the 
two poles, N S, of the 
magnet. Two screw-cups 
upon the stand communi- 
cate, one with the cup which supports the wire, and the other 
with the mercury-basin. 

Experiment. — Connect the screw-cups with the battery, so 
as to cause a galvanic current to traverse the w^ire, W. This 
will be attracted alike by the two poles of the magnet, and ag it 
can revolve around neither, owing to the counter attraction of 
the opposite pole, it w^ill be driven forward or backw^ard, accord- 
ing to the direction of the current, in a straight line between 
the two forces, to the positions indicated by the dotted lines ; 
and, upon leaving the mercury, the circuit will be broken, 
causing the wire to fall by its own weight back again into it, 
so as again to renew the circuit, and be again attracted. Thus, 
a vibratory motion of the wire will be maintained as often as it 
becomes the path of the galvanic flow. 




EflFect of submitting the conducting wire to the action of both poles of a 
magnet ? Explain Fig. 201. Why does the wire continue to -vibrate? 



SPUR-WHEEL. 



251 




ExperiTtient a. — A more interesting form of the last exper- 
iment may be shown by the 
Revolving Spur- Wheel, 
Fig. 202, where the vi- 
bratory movement is con- 
verted into one of rotation. 
Instead of a wire, as in the 
previous experiment, a 
spur wheel is so suspended 
between the poles of a 
magnet, or electro-magnet, 
that its points shall enter successively a small basin of mercury 
arranged on the stand, as in Fig. 202. 

Thus, during the passage of the galvanic current, each point 
becomes a conductor, and under the influence of the magnetic 
poles is driven forward and out of the mercury, just as 
the next succeeding one enters, and so causing the wheel to 
rotate, on the same principle that the wire in the last exper- 
iment was made to vibrate.* 

225. If an electric current be made to traverse a coil of 
loire free to move, this coil loill arrange itself at right 
angles with the poles of the earth, or those of an arti- 
ficial magnet. — As action and reaction between the elec- 
tric current and magnet are equal, when the latter is perma- 
nent, the wire conveying the former, if free to move, will 
arrange itself at right angles with the polar axis of the 
magnet. 

The position of such a coil, in reference to the earth's 

* As the points successively leave the mercury, and so break the galvanic cir- 
cuit, if in a dark room, sparks will be seen at the point of rupture, and, if these be 
repeated with suflBcient frequency, by an optical illusion the wheel will appear 
nearly at rest. If, instead of the spur-wheel, an entire disc of metal be sub- 
stituted, its revolution will be the same except more uniform. 



What results when the electric current is made to traverse a coil of wire free 
to move ? 



252 



BE LA RIVE'S RING. 



magnetic poles, may be illustrated by the arrangement seen in 

Fig. 203, and known as De La Rivers Ring. This consists of 

a compact coil of insu- 
lated wire, C, with its 
two ends attached to 
small plates of zinc and 
copper, Z, C. The coil 
with its plates rests in a 
glass cup, D, into which 
is poured sufficient dilute 
acid, to cover the plates : 
and the whole is allowed 
to float in a basin of 
water. The action of the 
acid on the metals will 
set free a current of elec- 
tricity; and this, flowing 
through the coil, will 

cause it to assume a definite position at right angles with the 

earth's magnetic axis, as previously stated. 

Thus, the two faces of the coil will take opposite polarities, 

like a magnet. If, while in this condition, the north pole of a 

magnet be presented to the south polar face of the coil, a 

mutual attraction will be exerted, as between the opposite pojes 

of two magnets, and vice versa. 

226. The polarity produced in a wire coil by the flow of n 

galvanic current through it may be shown by the Revolving 

Rectangle, Fig. 204. 

Here, a rectangular coil, C, is arranged in a vertical position, 

so as to revolve freely on two points between the poles of a U- 

magnet. 




Describe De La Rive's Ring. EflFect of a galvanic flow through the coil -whilo 
floating on the water? While the ring is in this condition, what results if 
the pole of a magnet be presented ? What does the Revolving Rectangle show ? 
How is this arranged ? 



REVOLVING RECTANGLE. 



253 



By means of the Pole- 
Changer^^ at P, the direc- 
tion of the current traversing 
the wire-coil is twice changed 
during each revolution, and 
thus twice changing the po- 
larity of the coil. These 
changes happen at the mo- 
ment when its axis is passing 
between the poles of the 
magnet, so that the condition 
of the wire in reference to 
the poles of the magnet 
causes it to undergo a con- 
stant series of attractions and 
repulsions just at those points 
where force is most needed to 
give it motion. 

Experiment. — Connect the Revolving Rectangle, Fig. 
204, with a Sulphate of Copper Battery, and, cause a 
galvanic flow through the wire-coil. This will immediately 
commence a rapid revolution, at a speed of from five to ten 
thousand turns in a minute ; showing the wonderful rapidity 

* The Pole- Changer attached to the coil may be illustrated by Fig. 205. This 
consists of two small semi-cylindrical pieces of silver, S S, fixed on the oppo- 
site sides of the axis of motion, A, but insulated from 
that and from each other ; to each of these segments 
is soldered one end of the wire composing the coil. 
The battery wires are terminated by horizontal por- 
tions of flattened silver wire, W W, which press 
slightly on opposite sides of the pole-changer, and must be so arranged, that 
the direction of the current flowing through the coil shall be reversed at the 
moment when its axis is passing between the poles of the magnet. — {Davis' 
Manual Mag.) 





State how the galvanic current flows through this so as to produce rapid 
motion. State the experiment with this. 

22 



254 



THERMO-ELECTRIC ARCH. 



with which electricity traverses the coil, in twice changing its 
direction during each revolution. 

227. We have already spoken of thermo-electricity, or 
that form evolved by the action of heat on two different 
metals (§ 211). This is subject to the same magnetic rccic- 
tions as galvanic electricity, which may be illustrated by 
the Thermo- Electric Arch, Fig. 206. This consists of a wire 
arch, mounted on a brass pillar, between 
the poles of a U-magnet. The lower 
or circular portion is of German silver, 
the upper of brass or iron. Upon a 
movable stand in front of the brtiss pil- 
lar is placed a lamp for heating the 
metals. 

Experiment. Apply the heat of a spirit- 
lamp to one of the junctions of the wires. 
A current of electricity will be made to 
flow from the German silver to the silver, 
passing up the heated side of the arch, 
and down the other, thus performing an 
electric circuit through this. This gives 
polarity to the wire, the same as to the 
coil, in Fig. 203, that face presented to 
the north pole of the magnet acquiring north polarity, and that 
to the south pole, south polarity. Repulsion of these, like 
poles and attraction between the unlike", causes the wire-arch 
to revolve half way round, which brings the other junction 
within the flame; the current is now reversed, and the face 
towards the north pole of the magnet acquires north polarity, 
and is again driven through a semi-revolution; thus a con- 
stant reversion of the electric current, flowing throug'h the arch, 
causes it to be alternately repelled and attracted by the mag- 
netic poles, and made to revolve. By placing the lamp on the 




Describe the Thenno-Electric Arch. State the experiments with this. 



THEORY OF THE EARTH' S MAGNETISM. 255 

Opposite side of the magnet, the direction of the current wil] be 
such that the south pole of the arch will be presented to the 
north pole of the magnetj and no revolution, consequently, is 
produced. 

228. Ampere's Theory of Magnetism. — From these, and 
a great variety of kindred illustrations. Ampere has deduced a 
theory, which supposes all magnetic phenomena to be produced 
by electric cm-rents. Thus, every molecule of a magnet is 
supposed to have a current of electricity perpetually circulating 
around it. The only difference between a magnet and a mere 
bar of iron consisting in the fact that, in the former, electricity 
is in a state of constant action around each ultimate particle of 
iron ; while, in the latter, this is in a quiescent or latent state. 
The resultant of these numerous little circuits in a magnet is 
the same as that of the electric current traversing a Avire-coil 
surrounding a bar of iron, as seen in Fig. 208. When the 
currents of electricity in different circuits move in the same 
direction they attract each other (§220), and, when in opposite 
directions, they repel. Hence, if the unlike poles of two mag- 
nets be placed end to end, the electric currents of each will be 
found moving the same way ; those of the north pole being but 
a continuation of those of the other, and thus the two poles 
will be drawn together; while, if the ends of like poles be 
presented, the course of the currents traversing each will be in 
opposite directions, and a repulsion will result. A magnetic 
needle tends to arrange itself at right angles with a wire trans- 
mitting an electric current, in order to bring the numerous 
currents circulating around its particles parallel with that of 
the wire. 

229. The magnetism of the earth is also explained, ac- 
cording to the theory of Ampere, by supposing the existence 

What does Ampei-e's theory of magnetisni suppose? DifTerence between a 
magnet and a mere bar of iron ? AVhat is said of currents of electricity moving 
in the same and in an opposite direction ? Wliy does a magnetic needle tend 
to arrange itself at right angles with the electric current ? How is the earth's 
magnetism explained by the theory of Ampere ? 



256 



THEORY OF THE EARTH'S MAGNETISM. 



Fig 207. 




of currents of electricity constantly traversing it near its sur- 
face, from east to west, in a direction at right angles with a 
line joining the magnetic poles. These currents are regarded 
as thermo-electric, and produced by the action of the sun's 
heat in his daily circuit. The action of such a current in 
inducing magnetism, and giving direction to a magnet, may be 
shown by the Terrestrial Helix, Fig. 207. If an electric 
current be made to circulate 
through thia coil in the same 
direction as about the earth, its 
action upon a magnetic needle will 
be similar to that of the earth, 
causing the needle, when stand- 
ing directly over the coil, to 
maintain a horizontal position to 
the axis, and to vary from this 
when carried towards the imaginary pole of a sphere, of which 
the coil represents the equator, the same as seen in Fig. 128. 

230. As we have already shown, temporary magnetism may 
be imparted to soft iron, by contact with a magnet ; we come 
now to speak of the manner of effecting the same by electricity. 
If a current of electricity he made to circulate around a 
bar of soft iron, it will render this magnetic so long as the 
current continues to floio. — The magnetic action of an electric 
current on a bar of iron varies with the number of revolutions 
it performs about this. Thus, a single turn affects the mag- 
netic needle, in Fig. 197, much more than the passage of the 
electric current on only one side of this ; so, by increasing the 
number of the turns of the conducting wire, the power of the 
current, in inducing magnetic properties in the iron, will be 
proportion ably increased. 

Experiment. — Let an insulated copper wire, loosely coiled, 



Effect of an electric current flowing around a bar of soft iron ? Effect on 
the magnetism of tlie bar by increasing the number of the turns of the wire? 



MAGNETISM FROM ELECTRIC CURRENTS. 



257 



Fig. 208. 




as seen in Fig. 208, be made the conducting medium of a 
galvanic fio^y, and place ^vitliin 
it a rod of soft iron. This rod 
will become instantly magnetic 
so soon as connection is made 
with the battery, having a 
north and south polarity, like 
a common magnet. Such is 

termed an Electro- Magnet. 

231. Soft iron, in such a position, instantaneously acquires 

and loses its magnetism whenever connection with the battery 

is made and broken. Hardened steel becomes less readily 

magnetic, but retains its magnetism after the current ceases to 

flow. 

Experiment. — With a larger and more compact coil or 

helix, magnetic effects may be produced much more striking. 

Fig. 209 shows such a magnetizing helix. A compact coil of 



Fig. 209. 




insulated copper wire is mounted on a stand, with its two ends 
terminating in the bottom of the screw- cups. Connect the 



Effect on soft iron in such a position ? On hardened steel ? Explain FJf 
209. 

22* 



258 



MAGNETISM FROM ELECTRIC CURRENTS. 



Fig. 210. 



poles of the battery with the screw-cups upon the stand be- 
neath, so as to cause the electric current to traverse the wire- 
helix, H, and a rod of soft iron placed within this will become 
instantly powerfully magnetic. This may be shown by attach- 
ing metallic bodies to its ends, as shown in the cut. The 
moment the flow of the current is interrupted by raising one of 
the polar wires of the battery slightly from the screw-cup, the 
iron rod ceases to be a magnet, and the suspended bodies fall. 

232. The wonderful effects of the electric current, in devel- 
oping magnetic power in small rods of iron, may be shown by 
the Magnetizing Helix ^ Fig. 210. This consists of a consider- 
able extent of wire, wound into a compact 
coil, with a small hole opening through the 
centre. 

Experiment. — Connect the ends of the 
wire with a battery producing a large flow of 
electricity,* and, with the helix in a per- 
pendicular position, as shown in the cut, 
drop through the opening a small iron tube 
or rod. This will become strongly magnetic, 
and be sustained within the coil without 
any visible support^ owing to the force with 
which it is drawn towards the middle of the 
coil by the opposite attractions. If the bat- 
tery and helix be of sufficient size, a con- 
siderable weight may be suspended, as shown 
by the figure. 

Such a helix, employed by Dr. Page in a 
lecture at the Smithsonian Institute, a few 




* The power of a common Pot Battery is insufficient to cause this experi- 
ment to succeed. A battery of much greater magnetic force, as the Trough 
Battery or Grove's, should be employed. 



Design of the Magnetizing Helix seen in Fig. 210 ? Give the experiment with 
this. 



ELECTRO-MAGNET. 



259 



Fig. 211. 



years since, was able to raise and suspend, free from any 
contact, a bar of iron which weighed eighty 
pounds. 

233. The induced magnetism of an electric 
current may be again shown by the wire coil, 
R, Fig. 211, and the semi-circles passing within 
this coil. These semi-circles are of soft iron, 
with faces evenly fitting to each other, and are 
provided with strong handles or rings. 

Experiment. — Connect the wires b and a 
with a Sulphate of Copper Battery, and then 
bring together the semi-circles, as shown in the 
figure. These, with a battery of medium size, 
will be held together with a prodigious force, 
sustaining a weight of more than a hundred 
pounds. After breaking the' flow 
of the current, they adhere 
slightly, but, lose entirely their 
magnetism when once separated. 
234. An electro-magnet of great power may 
be made, by bending a round bar of soft iron, 
ns shown in Fig. 212, and winding the arms 
with two or three layers of insulated copper 
wire.* When this wire is traversed by the 
electric current, such a magnet becomes far 
more powerful than an ordinary steel magnet 
of equal size. The power of such a magnet 




Fig. 212. 




* The magnetic power of the electro-magnet depends on the number of turns 
the wire takes around the bar ; thus, a small magnet, closely wound with fine 
copper wire, may be rendered far more powerful than a larger bar loosely wound 
with a coarse wire. The former magnets are used for telegraphs, shocking- 
machines, etc. 



Explain Fig. 211. Give the experiment. How may an electro-magnet of 
great power be formed ? 



260 



ELECTRO-MAGNETIC TELEGRAPH. 



may be shown bj the arrangement seen in Fig. 213, where 
the electro-magnet is fixed in a frame, and provided with a 
semi-circular armature, A, to which the weights to be suspended 




are attached whenever communication is made with the battery. 
This armature will be attracted to the magnet with a force 
sufficient to support a surprising weight. 

235. Electro-Magnetic Telegraph. — Among the wonderful 
triumphs w^hich science and art have achieved within the 
past few years, few hold so important a rank as the Electric 
Telegraph. For half a century previous to its application by 
Professor Morse, efforts were made from time to time to effect 
distant and rapid communications of thought by means of elec- 
tricity. In the earliest of these experiments the common 
Electric Machine was employed, and afterwards the Gralvanic 



Explain Fig. 213. What is said of the Electro-Magnetic Telegraph ? What 
was used for producing the electric current in the earliest experiments ? 



MORSE'S TELEGRAPH. 



261 



Batterj. Bj means of these, decomposition of Tvater and 
various chemical compounds were attempted for signals. The 
deflection of the magnetic needle was afterwards suggested bj 
Ampere, and, in 1837, introduced into practice on a large 
scale, by Wheatstone, in England. All these various devices 
for telegraphing, by means of electricity, yielded, however, to 
the beautiful simplicity and efficiency of the American Electro- 
Magnetic Telegraph, which is claimed to have been suggested 
by Dr. Charles T. Jackson and Professor Morse, in 1832, but 
was matured and practically introduced by the latter, between 
Baltimore and Washington, in 1844. 

236. The following description of Morse's Telegraph, 
abridged from Davis' Book of the Telegraph, will aid in 
comprehending the general operation of this wonderful con- 
trivance. 



Tig. 214. 







Fig. 214 shows the registering portion of the Electro-Mag- 



What afterwards ? What did Ampere suggest for giving signals ? What is 
said of Morse's discovery ? Describe the parts of Morse's registering appara- 
tus, shown in Fig. 214. 



262 



MORSE'S TELEGRAPH. 



netic Telegraph, along with the appendages usually employed. 
M is the electro-magnet, the wires of which connect with the 
two screw-cups, W w, upon the end of the stand. * L is the lever 
playing over the fulcrum, F, having an armature, A, at one 
end, near the poles of the magnet, while, at the other, is a 
blunt point, P, which strikes up against the roll, R, under 
which the strip of paper passes, and so marks it whenever the 
electro-magnet is in operation. This strip of paper, D D, is 
gradually drawn off from the spool, S, by means of two tight 
rollers at T, between which it passes. These rollers are 
moved by a clock-work arrangement beneath, at C, which is 
set in motion by the first movement of the lever. A bell, seen 
at B, is connected with the lever, so that upon the first motion 
communicated by the battery, this is struck, 4ind a signal thus 
given to the attendant. 

The battery by which' the register is worked may be 
twenty, fifty, or a hundred or more miles distant, provided, 
it be of sufficient power, and the wires conveying the elec- 
tric current properly insulated. f These wires are attached to 
the screw-cups, W w, of this register, causing the coil around 
the arm of the magnet, M, to form part of the circuit between 
the two poles of the battery. Thus, whenever the battery 
is in operation, and the circuit between its two poles com- 
plete, the electro-magnet, M, instantly becomes powerfully 
magnetic, and attracts the armature. This causes the point, 
P, to strike up against the paper, and leave a dot or a 
mark, varying in length according to the time the current is 

* This electro-magnet is wound with very fine copper wire, and contains 
usually some three thousand feet of this. In this way great power is given to 
tlie magnet, without rendering it of an inconvenient size. 

t Grove's Battery has been more generally used for working the telegraph; 
about thirty cups being required for a distance of 150 miles. These cups may 
be kept in one compact space, but operate the telegraph more successfully when 
distributed along the line. Such batteries will work for about two weeks with- 
out replenishing. Various other forms of batteries are adopted, as Bunsen's 
Charcoal, Daniell's Self-protecting, etc. Telegraphs, in some instances, are said 
to have been worked by the action of moist earth on metallic plates buried in it. 



morse's magnetic telegraph. 



263 



allowed to pass, and the armature to be attracted. Fig. 215 
shows a more modern and convenient form of the Telegraph 




Register, and one which is now generally used with Morse's 
lines. "^ 



* The'signs employed by Professor Morse are the following : These dots 
and marks are used to represent the various letters expressing the words or 
sentence transmitted : 



NUMERALS. 



u 

V 



z 

& - 



This combination of lines and dots is arbitrary, and may be changed at any 
time by an agreement between the telegraph operators. 



What is Slid of the battery by which the register is worked ? 



264 baine's chemical telegraph. 

The simplicity of the arrangement of the Morse telegraphic 
machines gives them some important advantages over other more 
ingenious and complicated forms described in the subsequent 
pages ; and hence their extensive introduction. Already these 
lines upon this continent extend over an aggregate distance of 
fifty thousand miles, and transmit, with the lightning's speed, 
messages throughout the length and breadth of the land. 

No recent invention has done more than the electric tele- 
graph to smooth the asperities which often exist between sec- 
tions widely remote, and strengthen the bonds of social union ; 
and to the discoverers of this wonderful method of communica- 
tion between distant cities, states and continents, civilization 
and Christian philanthropy may look as to important aids in 
their onward progress.* 

Baine^s Chemical Telegraj^h. — This differs from Morse's, 
just described, chiefly in the manner by which the message is 
registered, this being effected by the decomposition of a chemi- 
cal solution spread upon paper, which covers a circular tablet. 
This tablet is made to revolve by clock-work, while an iron pen 
or point writes in lines and dots the messages in spiral curves 
upon the paper of the tablet, f By means of the machines 
employed with this telegraph, messages may be transmitted 
with a rapidity considerably exceeding that of Morse's lines. 

* An ingenious application of the telegraph has been recently made to rail- 
way trains, by means of a battery and registering apparatus placed in these. 
A spring extending down from the register and battery bears gently upon an 
insulated wire, or other metal, laid between the rails, and thus an electric 
communication is afforded between the train and a distant railroad station, or 
between two trains. In this way an alarm may be given to or from a train in 
motion at any point on the road. 

t Experienced operators of this as well as Morse's machines often dispense 
with the paper altogether, writing the messages directly out from the sound of 
the clicks upon the plate or cylinder. 

What is the extent of these telegraph lines in this country ? What results 
has the invention of the Electric Telegraph tended to bring about ? What is 
said of Baine's Chemical Telegraph ? 



SIGNAL-KEY. 



265 




237. The Signal-key or Break-piece, Fig. 216, is the 

instrument usually em- 
ployed for interrupting 
the current and regulat- 
ing the system of lines 
and dots. This is placed 
near the battery, so as to 
be in the galvanic circuit. 
One ^yire from the bat- 
tery entering the screw- 
cup, P, while the other 

screw-cup, N, receives the wire of the telegraph which leads to 
the registering apparatus at the remote station. By pressing 
upon the knob attached to the spring, S, a connection is formed 
between P and N, and the electric current allowed to pass to 
the register, from whence it is returned to the battery again by 
a second telegraph wire. Thus the circuit is completed by de- 
pressing the spring, and broken again by the action of this 
when the fingers cease to press upon it. 

238. Instead of a second wire, directly connecting the regis- 
ter and battery, the earth is more generally employed as a con- 
ductor, by connecting the pole of the battery and register with 
a large metallic plate sunk in the ground at each terminus of 
the telegraph. In this case but a single wire is needed.* 

When the distance between the stations is great, the power 
of the electric current, owing to the imperfect insulation of the 

* These wires are more generally carried along lines of railroads, between 
distant points, and are elevated on strong poles above the danger of ordinary 
accident. A glass or stone ware support serves to insulate the wire where it 
rests upon the poles. In Paris, and other European cities, these telegraphic 
wires are enclosed in insulating tubes of gutta-percha, and laid under ground, 
thus rendering them less liable to accident, or to disturbances from changes 
of the atmosphere. 

Use of the Signal-key or Break-piece, Fig. 216? How is this used in operat- 
ing the register? Is it necessary that there be two wires for completing tlie 
circuit between distant stations ? 



266 house's printing telegraph. 

■wires, often becomes too feeble to operate successfully the 
registering apparatus/^ To remedy this defect, Professor 
Morse devised an arrangement called the Receiving Magnet^ 
whereby a second battery, placed in the circuit at the dis- 
tant station, is made to operate the register in the same 
manner as the first battery would do at a less distance. f 

239. House's Printing Telegraph. — This affords one of 
the most remarkable exhibitions of mechanical skill on record, 
and reflects the highest credit upon the inventive genius of Mr. 
Royal E. House, its distinguished, yet modest and unpretend- 
ing discoverer. This telegraph is operated by the agencies of 
electricity and condensed air ; • the former being required to 
compose and the latter to print the message transmitted. The 
constituent parts of this telegraph, as presented from an exter- 

*The Hughes^ Telegraph is an instrument recently patented by Mr. David 
E. Hughes, of Kentucky. This, like the House machine, prints the messages, 
employing for this, however, in place of the condensed air, an electro-magnet. 
The proprietors of this telegraph claim for it many advantages over those pre- 
viously invented. Among these may be stated the following : First, that the 
machine prints with greater rapidity, being capable of doing this as fast as the 
most expert operator can touch the pi'oper keys of the letter-boai'd. Second, 
that it allows of a self-locking of the machines intermediate between two prin- 
cipal stations, so as to transmit a message direct, as from Washington to Boston. 
Third, that it admits of transmitting messages in all states of the atmosphere. 
And, finally, that messages may be transmitted upon the same wire, in opposite 
directions, at the same movient. With a proper insulation it is said that a sin- 
gle battery will operate these machines at a distance of five thousand miles. 

We have recently examined a telegi'aphic machine, invented by Moses G. 
Farmer, Esq., telegraph engineer of this city, which, in point of simplicity 
of operation and cheapness of consti'uction, commends itself most fixvorably to 
our judgment. This machine prints plain Roman capitals, and may be oper- 
ated by an inexperienced person — the only requisite being an ability to read 
and spell. 

t Improvements in telegraphic apparatus now enable batteries to operate 
at \QYy great distances. A project is now on foot for extending a submarine 
telegraph across the Atlantic Ocean, from Ireland to Newfoundland, a dis- 
tance of 1G80 miles. Another telegraphic line is about to be extended from 



What is said of House's Printing Telegraph ? By what agencies operated ? 



HOUSE'S PRINTING TELEGRAPH. 



267 



nal view, are shoTvn by Fig. 217; this view can give the 
learner only a very general idea of its construction and oper- 
ation. 

240. The composiiig machine is arranged within a mahog- 
any case, C, while the 'printing arrangement stands upon this, 
and both are operated by turning a crank at A, or by a foot- 
power and treadle placed beneath the frame. At the front part 
of the case is a key-board provided with twenty-eight keys 

Fig. 217 




resembling those of a piano; on these keys are marked the 
letters of the alphabet and also a dot and a dash. Beneath 
the key-board revolves an insulated iron shaft, surrounding 
which is a cylinder provided with two spiral lines of points 
projecting up from its surface ; this cylinder revolves with the 
iron shaft passing through its centre, by means of a friction 
spring, and when pressed upon by a slight force may be 
stopped, while the shaft continues to revolve. 



London across the Mediterranean Sea, into Africa ; also to Greece, Con- 
stantinople, India, and so across to Australia. Thus 'n-e may soon expect a 
telegraphic communication to be formed with the distant regions of Asia and 
Polynesia, whereby messages shall be received from these opposite quarters 
of the globe in the space of a few minutes or hours at most. 



268 house's printing telegraph. 

To the end of this cylinder, beneath C, is attached a brass 
break- wheel, having fourteen teeth and as many intervening 
spaces ; upon these teeth strikes a spring, which connects with 
one of the battery- wires leading off to the distant station ; the 
other battery-wire passes from the magnetizing helix in the 
upright cylinder at E, and connects with the iron shaft at the 
end beneath D, thus causing this and the break- wheel to lie in 
the electric circuit. 

As the cylinder revolves, the current is rapidly interrupted 
by the spring and break-wheel, and the electric pulsations 
transmitted and made to give a corresponding revolution to a 
type-wheel of the machine at the remote station. When a key 
is depressed, it catches the pin upon the cylinder, as it comes 
round beneath, and so stops the motion of this until the letter 
upon the remote type-wheel corresponding with that upon the 
key is printed; this is then released, and another key de- 
pressed, which in like manner stops the cylinder until its 
letter is also printed ; and so the process goes on, the letters of 
the words of the message transmitted being rapidly printed at 
the remote station in plain Roman capitals, at the rate of two 
hundred and fifty or three hundred per minute. 

241. The Printing Machine. — This stands upon the 
mahogany case above the composing apparatus, and is worked 
by a manual power at A, independent of electricity. The 
variety of the parts composing this machine will allow of only 
a general description from a single figure, and the brief limits 
here assigned to this subject. 

Below the small dome, at P, is the escapement-wheel. This 
is a steel wheel about two inches in diameter, and revolves 
with the vertical shaft passing through the circular iron plate, 
by means of a friction arrangement. This escapement-wheel 
may be stopped, while the shaft to which it is attached contin- 
ues to revolve. Upon the lower circumference of this wheel 
are fourteen teeth, corresponding with those upon the break- 
wheel of the composing machine ; upon these teeth play the 
pallets of an escapement, F, having the end of its lever con- 



house's prixtijtg telegraph. 269 

nected Yrith the piston-rod of an air-cjlinder placed beneath 
the cii'cular plate at F. As this piston-rod moves once back 
and forth, it causes the escapement to vibrate and allow a tooth 
of the wheel to escape ; thus giving to this wheel a motion 
corresponding to that of the break- wheel of the cylinder below. 
Around the circumference of the escapement- wheel, above the 
escapement, are twenty-eight equi-distant projections, on which 
are engraved in order the alphabet, a dot, and a dash. This 
wheel, accordingly, advances one letter at each "vdbration of the 
escapement. 

Within the upright cylinder, at E. is a magnetizing helix 
placed in the electric circuit : in this helix are fixed several 
separate annular electro-magnets, which act upon correspond- 
ing armatures, fixed upon a rod supported at its upper ex- 
tremity by a horizontal spring passing across the elliptical ring, 
N, above the cylinder. As the electric current passes, the an- 
nular magnets within the helix become charged, and draw down 
the armatures and rod : this opens a valve coimecting with the 
upper portion of the rod, and admits the air from an air-cham- 
ber below to the cylinder which works the escapement. Thus, 
with every pulsation of electricity transmitted, an air- valve is 
opened, and a consequent vibration of the escapement and 
advance of the wheel and type is made, corresponding with the 
revolution of the cylinder and break-wheel of the composing 
machine. 

Placed on a pulley, at T, is a coil of paper, on which the 
messages are printed; when the machine is in operation this 
paper strip is di^awn ofi", and passes by a toothed cylinder be- 
tween the coloring-band, S, which revolves about this cylinder 
and the escapement or type wheel. Whenever the type-wheel 
makes a sufiicient pause (not less than one tenth of a second), 
by depressing a key of the key-board an eccentric wheel at Gr, 
through a connecting-rod, draws the toothed cylinder along 
with the paper and coloring-band against the type of the wheel, 
and so impresses a letter on the paper, 

242. If any desired letter on the type- wheel is placed in a 
23^ 



270 house's printing telegraph. 

certain position, and a corresponding key of the composing 
machine is depressed, by raising that key and again depressing 
it, the circuit-wheel at one station and the type-wheel at the 
other station all make a single revolution, which brings the 
letter around to its former position. Any other letter is 
brought to this position by pressing down its key, the circuit 
being broken and closed as many times as there are letters from 
the last one taken to the letter desired. 

Within the dome, at P, revolves the letter-wheel. This has 
painted on its circumference the letters to correspond with those 
of the keys below. In transmitting a message, each letter 
printed at the remote station is shown at a small opening in 
the front of this dome, so that, in case from any cause those of 
the keys do not correspond with those of the type- wheel, the 
disagreement is at once shown, and the type-wheel set to cor- 
respond. 

243. In transmitting a message, the machine is set in motion, 
a signal given, and then, with the communication before him, 
the operator commences to play, like a pianist, on his key- 
board, touching in rapid succession those keys marked with the 
consecutive letters of the message to be transmitted. On hear- 
ing the signal, the operator at the receiving-station sends back 
the signal, "ready," and then the communication is trans- 
mitted. 

The function of the electric current in this machine, together 
with the condensed air, is to preserve equal time in the printing 
and composing machines, that the letters in one may corre- 
spond with the other. The electric pulsations determine the 
number of spaces or letters which the type-wheel is permitted 
to advance; they must be at least twenty-five per second to 
prevent the printing machine from acting. The intervals of 
time the electric current is allowed to flow unbroken are 
equal, and the number of magnetic pulsations necessary 'to 

What is said of the Fire-Alarm or Municipal Telegraph ? 



FIRE-ALARM TELEGRAPH. 271 

indicate a different succession of letters are exceedingly un- 
equal : from A to B will require one twenty-eighth of a revo- 
lution of the type-wheel, and one magnetic pulsation; from 
A to A an entire revolution of the type-wheel and twenty- 
eight magnetic pulsations. 

The battery for operating this Telegraph is the same with 
that for Morse's Telegraph. 

244. The Fire- Alarm Telegraph. — This may be reck- 
oned among the most ingenious and useful applications of 
Electro-Magnetism yet devised, and is due to the genius and 
skill of Dr. W. F. Channing, of Boston, where it was first 
applied in 1851. 

The design of this Telegraph is to communicate simultane- 
ously to various quarters of the city, or town, an alarm in case 
of fire, riot, or other catastrophe, announcing, at the same time, 
the section in which these dangers exist. In this alarm-arrange- 
ment there are three prominent parts or divisions. First, the 
Central Station-^ where the batteries and various instruments 
of communication are placed; second, \hQ Signal Circuit oi 
wires for receiving and transmitting signals between the central 
station and various points ; and, third, the Alarm Circuit., by 
which an impulse is sent out from the Central Station to the 
machinery by which the alarm-bells are struck. 

In Boston the city is divided into seven Fire Districts. 
When a fire breaks out in one of these, the watchman or other 
authorized person in that district goes immediately to the 
Signal Box, which he opens with a key, and gives six turns of 
a small crank : this communicates the danger and the point 
whence it proceeds to the Central Station, from which an alarm 
is at once struck on the various alarm-bells, corresponding 
with that of the district ; thus the firemen or police are directed 
to the proper quarter. Should the fire or other cause of alarm 
be soon subdued, a signal, " all out," may be sent to the Cen- 



The design of this Telegraph ? The three prominent divisions ? State the 
manner in which an alarm is given in case of fire or other danger. 



272 



REVOLVING ARMATURE. 



tral Station, and the intelligence be immediately given through 
the same alarm-bells. 

The wires communicating with the signal stations and the 
alarm-bells pass on insulating supports above the buildings, 
free from danger of injury. The striking arrangement is 
similar to that of a common town-clock, and is set in motion 
by an electro-magnet worked by the battery at the Central 
Station, and a second local battery. 

The utility of the Municipal Alarm-Telegraph has been 

fully tested in most of the large 

^^* ^ ■ cities both in this country and Eu- 

J^ rope, and found to equal the most 

_j===J sanguine expectations of its project- 

ors. A full and complete description 
of this Telegraph, from the pen of 
Dr. Channing. will be found in the 
American Journal of Science, vol. 
XIII., Second Series. 

245. For illustrating the manner in 
which motion and power may be pro- 
duced by the electro-magnetic force, 
a variety of ingenious machines have 
been devised : a few of these We will 
describe.* 

The Revolving Armature^ Fig. 
218, consists of a bar armature. A, 
arrano;ed to revolve in a horizontal 




* Among the manufacturers of machines for illustrating the properties of 
Magnetism, Galvanism and Electro-Magnetism, may be mentioned Mr. Daniel 
Davis, junr., of Boston, who has earned an enviable reputation in these depart- 
ments of philosophical manufacturing. Mr. Davis has recently retired, and 
been succeeded by Messrs. Palmer and Hall, who continue the business suc- 
cessfully at their rooms, 158 Washington-street, and well merit the patron- 
age of the scientific public. 



Describe the Revolving Armature. Explain its operation, as given in the 
experiment. 



REVOLVING DISK. 



273 



plane just above the poles of an electro-magnet, fixed in a verti- 
cal position, as seen in the figure. To the axis of motion of 
this armature, at B, is affixed a break-piece. This is formed by 
filing away two opposite sides of the vertical shaft, so that the 
small silver springs at S shall bear upon it during only a por- 
tion of its revolution. One of these springs connects with one 
of the screw-cups, and the other, through the wire which 
surrounds the arms of the magnet, with the other. 

Experiment. — Upon connecting the poles, C Z, of the battery 
with the screw-cups, an electric current, flowing through the wire, 
strongly charges the magnet. If the armature be now turned 
so as to stand a little inclined from a right angle with the plane 
of the magnet, it will be attracted towards the poles of this ; 
but, on reaching these, the springs will cease to touch the shaft, 
and the current will thus be broken. The attraction of the 
magnet being now destroyed, the momentum of the armature 
will carry it forward little more than quarter of a circle, w^hen 
the springs will again touch, and the electric current again pass, 
causing the magnet once more to act, and drive forward the ar- 
mature. Thus, by a systematic series of breaks and connec- 
tions, great speed and considerable momentum may be acquired. 
246. Fig. 219 is a modification of the 
last instrument, designed for showing 
the efiect of rapid motion in blending 
colors. In this the axial shaft revolves 
on a support between the poles of the 
magnet, having the break -piece beneath 
the armature. Upon the end of the shaft 
above is fixed a thin pasteboard disc, upon 
which are painted the seven primary 
colors. 

Experiment. — Connect with the bat- 
tery, and turn the armature away from the 
plane of the magnet, as in the last experi- 
ment, when it will commence a revolution 



Fig. 219. 




What does Fig. 219 show? 



274 



RECIPROCATING ARMATURE ENGINE. 



carrying with it the disc so rapidly as to cause the seven colors 
to appear blended in one, viz., a brownish white. 

247. The Reciprocating Armature Engine^ Fig. 220, is an 




interesting contrivance for showing the reciprocating and rotary 
motion which may be imparted by the electro-magnetic influ- 
ence. Two electro-magnets, M M, are firmly secured in a ver- 
tical position, having their four poles appearing just above the 
small wooden table. Two armatures, A A, connected by a 
brass bar, move on a horizontal axis in such a manner that 
while one is approaching the poles of the magnet over which it 
is placed, the other is receding from those of the other magnet. 
The brass bar is connected with one extremity of the horizontal 
beam, B, which communicates motion by means of a crank to the 
fly-wheel, W.* Upon the axis of this wheel is a break-piece, f 
which regulates the magnetism of the magnets. 

Experiment. — Connect the screw-cups with the battery, 
and the electric current will traverse the wires of each magnet, 
causing them to become alternately charged, and attract the 

* By attachments to the beam, or to the shaft of the fly-wheel, motion may 
be given to a miniature saw-mill, or other amusing contrivances, 
t Davis' Manual. 



Describe the Reciprocating Armature Engine, Fig. 220. 
passage of the galvanic current give motion to this ?" 



How does the 



davexpqrt's electro-exgine. 275 

armatures, thus communicating a rapid reciprocating motion 
to the beam, and consequently a rotary one to the fly-wheel. 
248. Davenporfs Electro- Engine. — Fig. 221 is a form 



Fig. 221. 




of engine for multiplying the power produced by the revolution 
of the armature before the poles of an electro-magnet. In this, 
the communication between the two poles of the battery is 
through the wire of the magnet, G G, up the pillar for the 
break-piece, and along this to the shaft D, at E, along which it 
passes to a support between the arms of the magnet, where it 
flows down this support, and off* upon the other battery-wire. 
The flow of the current is broken by the spring at E, just as 
the armature, F, is moving by the poles of the magnet, similar 
to the machines previously described. 

A perpetual screw upon the shaft at C plays into the wheel, 
A, causing this to revolve comparatively slow, but with consid- 
erable power. By a drum fixed upon the shaft, B, motion is 
communicated to other machines. This, in principle, is the 
method by which mechanical power is obtained by the electro- 
magnetic force.* 

* It was formerly thought that this force might be indefinitely increased, 
and so serve as an efficient motive-power for propelling machinery. It is 
found, however, that the electro-magnetic power does not increase in a corre- 
sponding ratio with the size of the machine and expense of the materials em- 
ployed. No improvements will probably realize for this, motive-force once 
anticipated. 



276 



REVOLVING ELECTRO-MAGNET. 



249. The Revolving Electro-Magnet, Fig. 222, affords an 
instance of rapid motion produced b/ 
the reciprocal action of a common steel 
magnet and an electro-magnet. A 
common U-magnet, placed in a vertical 
position, has a small, straight bar 
electro-magnet, B, fixed to a vertical 
shaft so as to revolve between its poles.* 
The ends of the wire of this electro- 
magnet are soldered to two segments 
on opposite sides of the shaft, insulated 
from each other and the shaft. Two 
small silver springs, connecting with 
the screw-cups at the sides, press alter- 
nately on these segments (see Fig. 
205), ceasing their bearings while the 
electro-magnet is passing by the plane of 
the U-magnet. thus causing the polarity of the former at this 
point to be destroyed, and to be renewed again in a re verso 
direction, when the electro-magnet shall have moved along 
so as to bring the springs again in contact with the segments. 

By this means, the direction of the current through the wire, 
and, of course, the polarity of the bar, B, are twice changed 
during each revolution. Thus the position of the two magnets 
to each other is such, that, during the first quarter of the revo- 
lution, a mutual repulsion of like poles, and, during the second 
quarter, a mutual attraction of unlike poles, takes place, 
causing the bar and shaft to revolve by the action of four 
conspiring forces. 




* The bearings on •which these electi'o-magnets revolve are delicate, and the 
instrument, ■without proper care, may become easily injured and rendered 
inoperative. 



What does the Revolving Electro-Magnet illustrate? Describe the parts of 
this. State the direction of the flow of the galvanic current through the hor- 
izontal bar armature during its revolution. How does this change of the ttuw 
c.f the current act to give motion to the armature ? 



ELECTRICITY IN MOTION. 



277 



Experiment.— QoTmeci the screw-cups of the KeYolvinT 
Electro-Magnet with a Sulphate of Copper Battery, and turn 
the bar, B, slightly from the plane of the U-magnet, when an 
electric current will pass through the wire and springs, between 
the two poles of the battery, causing the bar and shaft to re- 
volve with surprising velocity.^ 

ri^.223. ^" ingenious modification of this 

instrument has a bell arrangement at- 
tached, as shown in Fig. 223, whereby 
the rate of revolution of the bar and 
shaft may be accurately measured 

250. Wires conducting electric 
cnrrents, if free to move, attract each 
other when the currents are moving 
in the same, and repel each other 
when moving in the opposite direc- 
tion, —^^q tave seen (§ 151) that 
bodies charged alike with electricity 
at rest, repel, while, charged unlike, 
they attract each other. Such is not 
the case with electricity in motion, 
since currents of the sa7ne electric- 
ity, moving in the same direction, 
attract each other. 

* These electro-magnetic machines for showing motion, merely should be 
worked with a battery of low intensity. The Sulphate of Copper Battery is a 
convenient form. An instance, a few years since, came to the knowledo-e of 
the author, where an operator, ignorant of the operations of the galvanic" bat- 
tery melted down the springs of the pole-changers of several valuable 
machmes, by connecting with a large trough battery when in operation 

Care should be taken to have the springs of this pole-changer press upon 
the segments with just as much force as is necessary to make and cease their 
bearmgs at the proper points. By a proper adjustment of these springs, the 
bar and shaft may be made instantly to reverse their revolution by simply 
changmg the polar wires from one cup to the other of the battery 




What is said of wires conducting electric currents, and free to move? 
2i 



278 



CONTRACTING HELIX. 



Fig. 224. 




The Contracting Helix^ Fig. 224, will serve to illustrate 
the attractive power of elec- 
tric currents, moving in the 
same direction. A wire loose- 
ly coiled is supported in a 
vertical position at its upper 
extremity, on the top of a 
brass pillar which connects 
with one of the screw-cups, 
while its lower end just dips 
in a cup of mercury, which 
connects with the other screw- 
cup. 

Experiment. — Connect 
the poles of the battery with 
the scrcAv-cups, and the elec- 
tric current, traversing each 
coil of the wire in the same 
direction, will cause these to be drawn together, and the 
helix to become thus shortened; this will lift the lower end 
from the mercury, and interrupt the current, when the helix, by 
its elastic force, will be again lengthened, and enter the mercury ; 
this will allow the current to pass once more, and contraction will 
again take place, and thus a rapid succession of vibrations will 
be kept up so long as the current from the battery is allowed to 
pass. As the extremity of the wire is lifted from the mercury 
at each contraction a brilliant spark may be seen accompanied 
by a slight report. One end of a bar magnet passed down 
into the helix, as seen in the cut, will render the vibrations 
much more rapid and energetic* 

251. Whenever an electric current flows through a wire^ 
it excites another current in an opposite directio?i, in a 

* See that the wire does not enter the mercury so far as not to be lifted en- 
tirely out of it by the contraction. 

When a cuiTent cf electricity traverses the wire of the Contracting Helix, 
Fig. 2:24, why are the circles drawn together, and the perpendicular length 
of the coil shortened ? Give the proposition section 251. 



SECONDARY CURRENT. 



279 



second loire held near to and i^arallel with it; and^ on sud- 
denly interrupting the first current^ the second or ifiduced 
one instantly reappears in an opposite direction to the course 
it first followed. — This proposition may be illustrated by the ar- 
rangement shown in Fig. 225, known as the Separable Helices. 
This is composed of two helices of insulated wire, fitting one 
within the other, but entirely separate. The inner one, F, 
formed of coarse copper wire, is fixed in a vertical position on 

Fig. 225. 




the base-board ; one of its ends connecting with the screw-cup, 
A, and the other with the rasp, B. The exterior helix, E, is 
composed of a great extent of fine insulated wire, which may be 
lifted off from the inner helix when desired. Its ends are en- 
closed in two brass caps, to which are soldered the extremities 
of the wire. Attached to these caps are the screw-cups, C and 
D. A bundle of annealed iron wires, G, is placed within the 
inner helix, and may be removed at pleasure. 



Describe the construction of the Separable Helices, Fig. 225. 
periment with the Separable Helices. 



Give the ex- 



280 SECONDARY CURRENT. 

Experiment. — Attacli one pole of a battery to the screw- 
cup, A, and draw the end of the other over the rasp ; bright 
sparks will be seen at each interruption of the current, and if 
the metallic handles, H, connecting with the outer helix, be 
grasped bj the hands, shocks will be felt in the wrists and arms, 
as the wire is passed along the rasp. 

252. Thus the j)rimary or battery current, which flows 
through the inner helix, induces a secondary current in the 
outer. This secondary current is momentary in its action, and 
appears only at the opening and closing of the primary circuit, 
and is more intense in its effects than this latter. The current 
excited in the outer helix by the closing of the primary circuit, 
is called the 'initial secondary, and that upon the opening of 
this, the terminal secondary. These two currents floAV in op- 
posite directions, and always contrary to the course of the pri- 
mary current, as may be shown by the deflection of the needles 
of two delicate galvanometers, placed in the circuit of each current. 

253. The action of the terminal secondary is mtch more 
intense than that of the initial. This may be shown by con- 
necting the wire of the inner helix, Tig. 225, with a cup of 
mercury instead of the rasp. Upon closing the primary cir- 
cuit, by plunging the end of the wire leading to the battery in 
the mercury, the shock produced by the initial secondary cur- 
rent, thus formed, will be found much less severe than that from 
opening the circuit when the wire is raised from the mercury. 

Experiment. — While the primary current is flowing through 
the inner helix, and the handles are grasped as in the last ex- 
periment, slowly introduce into the vertical opening the bun- 
dle of wires, G. The intensity of the shocks will be greatly 
increased, and, when the wires are fairly entered, will often be 
too severe for endurance. 



Explain the flow of the primary and secondary currents in this experiment. 
IIow do the terminal secondary and initial cmn*ents compare in their effects ? 
ITow may this be shown ? What is the effect of introducing a bundle of wires 
into the inner helix while the galvanic current is flowing through it ? 



COMPOUND MAGNET AND ELECTROTOME. 



281 



Fig. 226. 



These wires become powerfully magnetic, and react on the 
electricities of the two coils, so as to increase the energy of 
their action ; the withdrawing of even a single wire from the 
bundle perceptibly affecting the power of the two currents. 
If, instead of the wires, a solid bar of soft iron be introduced 
into the opening, a similar but less powerful reaction will be 
produced. 

254. The shocking or decomposing effects of the secondary 
current are much greater when the interruptions of the primary 
current are frequent. To ensui^e a rapid succession of breaks 
and contacts various plans have been proposed, as the rasp, al- 
ready mentioned, ratchet, and cog-w^heels moved by clock-work, 
etc. ^one have, however, effected this with greater convenience 
and efficiency than that where an armature moved by a tempo- 
rary magnet is employed. 

255. The Compound Magnet and Electrotome^ Fig. 226, 

is one of the most amus- 
ing and efficient instru- 
ments yet devised for 
showing the action of 
these secondary cur- 
rents. Two helices in- 
closing a bundle of 
wires, similar to the ar- 
rangement in Fig. 225, 
are placed in a horizon- 
tal position, and held 
firmly to the base -board 

by two brass bands. The screw-cups, A D, receivenhe battery 
connections. From A leads a wire, connecting with the band 
which supports the mercury-cup, C, while to D is soldered one 
end of the inner or primary coil, the other end of which is con- 
nected with the band on which is fixed the small mercury-cup, 

How do frequent interruptions of the primary affect the action of the 
secondary currents ? 

24* 




282 COMPOUND MAGNET AND ELECTROTOME. 

B. The bent wire, W, is arranged to vibrate up and down 0:1 
the horizontal axis, II, while its two points, at B and C, just 
dip into the mercury of these cups. The curved iron rod, R, 
attached also to H, is bent, so that its lower extremity shall 
approach quite near the inclosed bundle of wires. For regu- 
lating and giving a proper balance to this vibratory arrange- 
ment (which should preponderate slightly towards B and C), a 
small ball, 6, is made movable on a wire screw attached to the 
axis. To E F are soldered the ends of the wire composing the 
outer helix, to which are attached also metallic handles for 
receiving shocks. 

Experiment. — Connect A D with the battery, and the 
electric current will traverse the inner coil and the bent wire, 
W ; the bundle of inclosed wires will become instantly mag- 
netic, and attract R ; this will lift the points of W, at B and C, 
out of the mercury, and so break the current. The magnetism 
of the bundle of wires, by which R was attracted, being thus 
destroyed, the points will again fall into the mercury, and so 
the flow of the current be renewed, and R again attracted. 
Thus, a series of rapid vibrations, interrupting the flow of the 
primary or battery current, will produce a violent action of the 
secondary. If shocking-handles attached to E F, as in Fig. 227, 
be now grasped with moistened hands, the shocks will become 
quite intolerable, causing them, by an involuntary contrac- 
tion of the muscles, to clench the handles too firmly to be 
easily released. As the points leave and enter the mercury in 
B and C, brilliant sparks will be seen, accompanied by sharp 
snaps.* 

256. Experiment. — Place one of the handles. Fig. 226, in a 
glass basin of water, and let a person grasp the other with one 
hand, while with the other he attempts to remove from the basin 
any object, as a coin. A violent shock will be felt the instant 

* The wire, W, should be nicely balanced, and the points but just enter the 
niercury. 

Give the experiment with the Compound Magnet and Electrotome. 



MAGNETO-ELECTRICITY. 



283 



the fingers touch the water, causing a sudden withdrawal of the 
hand. This will be repeated as often as the attempt is made, 
to the great amusement of the spectators. 

By removing the handles, and in their place attaching to the 
wires small strips of platinum, the various experiments in de- 
composition of liquid compounds (§202) may be performed 
as with the primary current. 

257. Magneto- Electricity is the name given to electricity 
produced by the action of a magnet. As electricity, flowing 
through a wire surrounding a bar of soft iron, induces magnet- 
ism in it, so, on the contrary, a magnetized bar sets in motion 
a current of electricity in such a ware surrounding it. This 
may be shown, by introducing one of the poles of a powerful 
bar magnet within a helix of fine insulated wire, the ends of 
which are connected with a delicate galvanometer. The flow 
of an electric current through the wire will be perceptible, by 
the deflection of the needle, as the magnet enters and leaves 
the helix, the direction of the current changing with the poles 
entered. 

Fig. 227. 




258. The Magneto-Electric Machine, Fig. 227, is a con- 



What is Magneto-Electricity ? How is this form of electricity produced ? 
Describe the Magneto-Electric Machine. 



284 MAGNETO-ELECTRICITY. 

venient arrangement for developing electricity by the reaction 
of a magnet. A bar armature, G, of soft iron, bent twice at 
right angles, is made to revolve rapidly before the poles, N S, 
of a powerful compound magnet, by means of the multiplying 
wheel, Jj which belts off upon a small drum attaclied to the 
axis of the armature. The arms of this armature are wound 
with a continuous insulated wire, the ends of which are soldered 
to the two segments of a pole-changer attached to the axis as 
in Fig. 204. Two small springs, pressing alternately on these 
segments, connect through the wires, E B, with the screw-cups 
at the end of the base-board. 

Upon causing the armature, G, to revolve, as its ends come 
directly before and near to the poles of the magnet, this arma- 
ture becomes itself strongly magnetic. This sets in motion the 
natural electricity of its helices, which flows in a certain di- 
rection, and is conveyed through the springs and wires, to the 
screw-cups. As the armature moves on, when at right angles 
wdth the plane of the magnet, it loses its previous magnetism, 
and begins to acquire a new charge of an opposite polarity ; 
this excites in the helices a new current in a reverse direction ; 
this, by means of the pole-changer, is turned in the same 
direction as the former current, and conveyed to the screw- 
cups. Thus, the magnetism of the armature is twice changed 
(luring each revolution, exciting in the helices two electric cur- 
rents, flowing in opposite directions. 

By attaching to the screw-cups the shocking-handles, p n, 
and causing the armature to revolve rapidly, shocks similar to 
tliose in Experiment, § 255, may be received. The Magneto- 
Electric Machine has been also successfully employed mth. the 
Telegraph, and as a substitute for the Galvanic Battery in Elec- 
tro-Metallurgy. 

259. Few departments of natural science possess, at present, 
more general interest than those of Galvanism and Electro- 
Magnetism ; being connected, as they are, so extensively, with 

Explain the manner in which electricity is produced by this machine. 



GENERAL REMARKS. 285 

Avonderful and important phenomena. The researches and prac- 
tical discoveries of Faraday, Henry, and others, have done 
much of late to call attention to these sciences, and disclose a 
fruitful field for investigation, as yet but partially and imper- 
fectly explored. 

The applications of galvanic electricity, since it was first dis- 
covered, are of the most varied and wonderful character. By 
it the telegraph operator can write, with his iron pen, a letter, 
three thousand miles distant. By it the electrotypist coats the 
most common metals with silver and gold; by it metals are 
extracted from aqueous solutions and even from the human 
system. It is employed to plate medallions, busts, jewelry, 
and even the very type which prints the words we write. It 
can produce the most intense and brilliant of artificial lights, 
and, in its blaze, the diamond and the hardest of metals become 
as wax. As a motive power, galvanic electricity has not yet 
succeeded to that extent which promises its introduction to any 
considerable degree in the propelling of machinery. That it 
may not yet be successfully applied for such purposes, we dare 
not in this age of inventions positively say. 

In the previous chapters we have attempted only a brief out- 
line of these subjects, and for more extended information the 
student is referred to Smees' Electro-Metallurgy, and Davis' 
Manual of Magnetism, a thorough and practical treatise, well 
deserving a careful perusal. 

Wliat is said of the interest now attached to Galvanism and Electro-Magnet- 
ism ? Why this interest ? "What is said of the applications of galvanic elec- 
tricity since its discovery ? Give some illustrations. What is said of this as a 
motive fjower ? 



286 THEORIES OF LIGHT. 



LIGHT. 

260. Light is that mysterious physical agent by which the 
eye perceives external objects. Of its essence we know nothing, 
and can only judge of it by the effects which it produces. 

Two prevailing opinions exist in regard to the nature of 
light; one, known as the Newtonian Theory^ regards it as 
composed of infinitely minute atoms of matter thrown off from 
luminous bodies, and impinging on the organs of vision to pro- 
duce sight, the same as odorifer9us ejffluvia on the nasal organs 
to produce the sense of smell ; the other, called the Undulatory 
theory^ supposes light simply as the result of undulations or 
waves excited by luminous -bodies in an exceedingly subtile 
medium called ether, and which traversing this medium pro- 
duces on the eye effects analogous to the vibrations of air on 
the ear in causing sound. =* 

Light moves with a velocity of about 192,000 miles per 
second; thus requiring only eight minutes. and thirteen seconds 
to pass from the sun to the earth, a distance of ninety-five 
millions of miles. Some idea of its surprising velocity may 
be gained by comparing its rate of motion with the graatest 
speed yet acquired by the locomotive engine (about one mile 
per minute). To traverse the distance passed over by light 
from the sun in eight minutes and thirteen seconds, such a 
body would require nearly one hundred and eighty years. 

261. Self-luminous Bodies or Luminaries. — Such are 

* As the limits of this work do not allow of considering these theories, the 
student is referred to Muller's Physics, sect. v. chap. v. ; Bird's Natural 
Philosophy, chap, xxi., and Brewster's Optics, in Lardner's Cyclopedia. 

What is light ? What do we know of the nature or essence of light ? How 
many theories exist in regard to the nature of light ? ^tate the Newtonian 
theory. The Undulatory theory. At what velocity docs light move? 
How does the speed of light compare with the fastest locomotive ? What are 
self-lummous bodies ? 



LIGHT. 287 

the original sources from whence all light proceeds. Of these 
the sun, a lighted candle, and phosjDhorescent bodies, are ex- 
amples. Such dispense their light in every direction from 
myriads of luminous points extending over their surfaces, and 
thus render visible the various objects on which their rays may 
fall. Bodies not self-luminous are either opaque, transparent, 
or translucent. 

Opaque bodies are such as wholly intercept the passage of 
light, and are visible only by the presence of self-luminous 
bodies. Thus, the moon, the planets, and most objects on the 
earth, are opaque, and, when placed between the eye and a 
luminous source, so intercept the light from this as to render it 
invisible. 

262. Transparent bodies are those which afford a passage 
for light sufficiently free to allow of our seeing distinctly the 
forms of objects placed behind them. These differ in the 
degrees of their transparency. Some, like thin plate-glass and 
portions of air. which are wholly invisible, are said to be 
perfectly transparent; others, as the better qualities of 
window-glass and most crystals, which are themselves visible, 
yet allow objects to be distinctly seen through them, are called 
transparent^ merely : while those bodies through which ob- 
jects are indistinctly seen, as ground-glass, clouds of smoke, 
etc., are said to be semi-transparent. 

Translucent bodies are those which allow a mere glimmer- 
ing of light to pass them, insufficient to show either the color 
or form of an object. Such, for example, are plates of horn 
and colored shell. 

A ray of light is simply the line which light makes in its 
progress through space. A pencil of light is a collection of 
rays diverging from, or converging to a point. When a col- 



Give examples. How do these dispense their light ? What are opaque 
bodies ? Give examples. What are transparent bodies ? Examples ? What 
are translucent bodies ? Examjjles ? Define a ray of light. A pencil of 
light. 



288 SHADOW AND PENUMBRA. 

lection of rajs proceed from a luminous body in parallel lines 
it is termed a beam of light. 

263. TAglit j)roceedhig from a lumwous point moves in. 
straight lines so long as the medium, ivhich it traverses is 
uniform. — As a consequence of this, if an opaque body be 
presented to the light from a luminous source, it will intercept 
those rays thrown upon it, and cast a shadow on the side oppo- 
site the light. The form of the shadow depends on the shape 
and relative size of the luminous and opaque bodies. Thus, if 
the light proceed from a point or luminary smaller than the 
opaque body, the shadow will divei-ge as it recedes from this 
opaque body ; but if the luminary be larger than the opaque body, 
the shadow 'will converge, and terminate in a point at a greater 
or less distance from the body. 

264. Shadoiu and Penumbra. — If the luminous body con- 
siderably exceed in size the opaque, there will be formed, 
besides the true converging shadow, a half shadow, or penum- 
bra, on each side of this, as seen in Fig. 228. Here S is the 

Fig. 228. 




luminary, and a smaller opaque body, behuiu wuiuu is lormed 
the true shadow terminating at t. From the space occupied by 
this true shadow the whole of the light from S is excluded, but 

A beam of light. How does light from a luminous body proceed? What is 
said of the form of the shadow ? What will be formed when the luminary ex- 
ceeds in diameter the opaque body ? How is the penumbra produced, and 
how does it differ from the true shadow, as shown by Fig. 228 ? 



PHOTOMETER. 



289 



on each side of this is a space, P P, which receives a portion 
of the rajs from S, and is called the penwnbra. Near the 
outline of the two shadows is quite clearly defined, but further 
from this, towards t. thej become less distinctly marked, until 
near ^, where the true shadoAV terminates, they become well- 
nigh blended. Beyond the point at t the true shadow ceases, 
and the penumbra goes on, growing wider and more faint, until 
it disappears in the distance. 

265. The iyitensity of light diminishes as the square of 
the distance from the luminary increases. — Thus, light ob- 
serves the same laws as gravitation, sound, and other radiant 
forces, varying in intensity with the extent of surface over 
which a given portion spreads itself. Accordingly, if a given 
surface, at the distance of one foot from a candle, receive a cer- 
tain number of rays of light, the same surface, removed to a 
distance of two or three feet, would receive four or nine times 
less number of rays. In other words, the intensity of the 
illumination at a distance of one foot from a single candle 
would be the same as that from four or nine candles at a dis- 
tance of two or three feet, these numbers being the squares of 
two or three, the supposed distances from the candle. 

266. The Photometer^* an instrument for comparing the 
intensities of light from different sources, has its action based on 
the above proposition. These are of different forms. Fig. 229 

Fig. 229. 



* Photos, light, and mdron, a measure. 



How does the intensity of light from a luminary diminish ? Give an illus- 
tration of this proposition in the case of a candle. What is the Photometer ? 

25 



290 sun's ligut. 

shows, however, the most simple arrangement of the Pho- 
tometer. A B is a vertical white screen, near to and directly 
in front of which is placed a perpendicular rod, E,. From this 
rod two shadows, S, S', are cast on the screen by the candles, G, 
L, these shadows just touching without overlaying each other, 
and each being illumined by the light forming the other shadow. 
If, now, a perceptible difference exist in the two shadows, and S, 
formed by G, be the darker, remove the candle, G, further from 
the screen, keeping the shadows in the same position, until both 
be equally illumined. By comparing the squares of the dis- 
tances of the two candles from the screen, when their positions 
are thus changed, the comparative intensities of the lights from 
these may be ascertained. Thus, if L be now 2 and G 3 feet 
from the screen, the intensities of their lights will be to each 
other as 4 to 9. 

267. The intensity of the slut's light exceeds that from 
any other luminary with ivhich we are acquainted. — The 
illuminating power of a light depends not only on its absolute 
intensity, but also on the extent of the luminous surface from 
which it radiates. Thus, the vast area of the sun, radiating to 
us light from a hemisphere of nearly a million and a half 
square miles, causes the intensity of his illumination to exceed 
by far the light from the brightest artificial lights that can be 
formed.* Even the intense brilliancy from charcoal points, 
when acted on by the galvanic current, § 270, is found inferior 
to that from solar light ; and, according to Dr. Wollaston, it 
would require more than 5,500 wax candles, at a distance of one 
foot, to equal the intensity of the sun's light. 

* The light from the moon is found to be 801,072 times less intense than that 
from the sun. Few persons are able from common observation to judge with 
any degree of accuracy of the comparative intensity of the light from different 
luminaries. 

Show from the figure how the degree of light from a luminary may be 
measured. What is said of the light from the sun ? Why is the light from 
this so intense ? How does the sun's light compare with the most intense arti- 
ficial lights ? 



REFLECTION OF LIGHT. 291 

EEFLECTION OF LIGHT. 

268. When light falls on any opaque body, a portion of it is 
absorbed, and the remainder turned back or reflected from the 
surface of the body. The amount and direction of this reflected 
light depend on the nature of the surface on which the light 
falls. If the body have a regular and highly-polished surface, 
nearly all the light will be reflected in a certain definite order ; 
but if its surface be irregular and unpolished, only a small por- 
tion will be thrown back, and that in the most irregular and 
confused manner. The former is an instance of regular^ and the 
latter of iiTegidar reflection. It is by the irregular reflection of 
light that most objects in nature become visible ; and since this is 
thrown off, in all directions, from the irregular surfaces of ma- 
terial bodies, it renders luminous and visible those not reached 
by the direct light from luminaries.* 

269. When light falls on a plane and polished surface^ 
as a TTiiiTor^ it follows the same laios of reflection as solids 
(§ 21), its angles of incidence and reflection being equal. — 
Let i Mj Fig. 230, be the direction of an incident ray of light, 

falling on the mirror, m m, and 
^^e-230. n r its course when reflected 

from this. Upon drawing a 
perpendicular, P ?/, to the plane 
at the point of reflection, it will 
be found that the angle of 
incidence, i 7^ P, is precisely 
equal to that of reflection, r/i P. 

* It is the reflection and refraction of the sun's light by the atmosphere and 
the bodies -which are suspended in it that produces the agreeable twilight, 
and so causes the transitions from day to night, and from night to day, to be 
gradual. 

What is said of light which falls on an opaque body ? What is an instance 
of regular reflection of light ? Of irregular reflection ? How do most objects 
in nature become visible ? What laws does light reflected from a plane surface 
follow ? Explain Fig. 230. 




292 



PLANE REFLECTORS. 



This same law holds good in regard to every form of surface, 
curves as well as planes, since the former may be supposed 
formed from an infinite number of minute planes. 

270. Rays of light falling on a pkuie mirror are reflected 
from it at the same angle as they apj)roached it, and ap- 
pear to j^roceed from p>oints just as far behind the mirror 
as those from which they issued were before it. — This is in 
accordance with the proposition previously stated, and may be 
illustrated by Fig. 231. Let M M be a plane and polished mir- 



Pig. 231. 




ror, on which are incident the parallel rays, P P', the diver- 
gent, D, and the convergent rays, c c c. The first are reflected 
back to/? p parallel ; the second, diverging from D, are thrown 
back at the same angle to d d d, and appear as if diverging 
from D', a point behuid the mirror ; the third, converging from 
c c c^ are reflected, converging to the point C, as though com- 
ing from c c c', behind the mirror.* 

271. If the object form an angle with the mirror, it will 
form, double that angle with its imaye. — Let A B be an 

* Mirrors are reflectors formed by coating the backs of polished glass plates 
vrith a brilliant amalgam of tin and quicksilver, while speculums are reflectors 
made from highly-polished metals. The latter aflord the more perfect reflect- 
ors, being used for telescopes and other optical instruments. 

State Proposition 270. Explain Fig. 231. If an object form an angle 
with a plane mirror at what angle will an image appear ? 



PLANE REFLECTORS. 



293 



Fig. 232. 




object inclined to the mirrorj M M', Fig. 232, and forming with 

it the angle B M' M ; then 
will the image A' B' be in- 
clined to the mirror at the 
same angle, B' M' M; the 
sum of these angles, B M' 
B', will be, therefore, the 
angle which the image makes 
with the object, and double 
the angle which either 
makes with the mirror. 
Accordingly, the image from 
an object in a horizontal position, formed by a mirror inclined 
at an angle of 45°, will appear erect. If the mirror be hori- 
zontal, and the object in a vertical position, the image from 
such object will be inverted. Hence it is that the images of 
trees and other objects bordering on a smooth sheet of water 
appear inverted. 

If an object be placed between two plane parallel mirrors, a 
series of objects will be produced, lying on a straight line 
drawn through the object perpendicular to the reflector. This 
is seen in rooms having mirrors placed parallel on opposite 
sides of the room, with a lustre or other object suspended be- 
tween them. An interminable range of lustres will appear in 
each mirror, which lose themselves in the distance by reason of 
their faintness. This increased faintness is caused by the 
repeated reflections, which diminish in each successive reflec- 
tion the amount of light, causing the objects to appear to 
recede in the distance.* 

272. If two 'plane mirrdrs be inclined towards each other 
at any angle^ images from an object placed between them 
will be multiplied according to the degree of the angle. — 

* Lardner. 



Why do trees and other objects bordering on a smooth sheet of water appear 
inverted ? State the proposition, section 272, in regard to two mirrors inclined 
to each other at any angle. 

25* 



294 KALEIDOSCOPE. 

This may be shown by the arrangement seen in Fig. 233. 
Let V Mj and 11 M, be two mirrors, forming with each 



Fig. 233. 




other an angle of 90°, and an object placed between. 
The eye at E will see not only the object at 0, but also its 
images at a and a', caused by a reflection of the rays proceed- 
ing dii'ectly from 0. Another ray from falling on H M, 
will be reflected to V M, from which it will undergo a second 
reflection, and meet the eye, causing a thii'd image at a' . Thus, 
with the mirror inclined as in the figure, the eye at E will see, 
besides the object itself, three images from as many difierent 
points. If the mirrors be inclined to each other at an angle of 
45°, seven images of w411 be seen, which, with 0, will be 
arranged at eight angles of a regular octagon, of which the 
point, M, where the mu-rors meet, will be the centre. By giv- 
ing a still greater inclination to the mirrors, the number of 
images will be proportionably increased ; and, if the angle of 
inclination be an aliquot part of 360°, the images will be ar- 
ranged on the sides of a regular polygon. 

If the mirrors be placed so as to form witli each other an angle of 90°, how 
many images of the object -will appear ? If inclined at 45-, how many images ? 
When the mirrors are inclined at an angle which is an aliquot part of 360^, 
how will the images be arranged ? Wliat optical toy acts on this principle? 



CONCAVE REFLECTORS. 295 

Upon this principle rests the construction of the kaleido- 
scope^ which consists simply of two pieces of common looking- 
glass, arranged in a tube at a certain angle, between which are 
loosely placed semi-transparent bodies, of various colors, to be 
reflected. 

273. Curved Reflectors. — If a segment of a hollow sphere, 
whose inside surface is brightly polished, be cut off by a plane, 
this segment will form a concave mirror. Such a mirror tends 
to collect the rays of light thrown upon it, and bring them to- 
gether. The point where the rays reflected from a concave 
mirror meet is called the focus (fire-place) of the mirror. 

Parallel rays of light falling on a concave mirror^ near 
the axis, are reflected to a focus at a point halfway betiueen 
the vertex and centre of the sphere described by the mirror. 
— Let r a ?', Fig. 234, be parallel rays proceeding from 

a distant luminarv, as 

Fig. 234. '^ 

the sun, and falling on 
the spherical concave 
mirror, ]M M' ; these rays 
will be converged to a 
focus at F, equidistant 
from the vertex, Y, and 
the centre, C, of the 
sphere described by the mirror. 

This focus is called the principal focus ; when the arc or 
aperture of the mirror exceeds about five or six degrees on each 
side of its axis, the parallel rays falling on the parts without 
this limit are converged to a point nearer the mirror than the 
principal focus, producing an aberration of the reflected light. 
To prevent this, and converge to a single focus all the rays 
that fall on the mirror, those with parabolic curves are con- 
structed. Such are known as burning mirrors. 

"What is a concave mirror ? What is said of rays of light falling on a concave 
mirror ? How will parallel rays of light foiling on a concave mirror, near its 
axis, be reflected ? What is the point where these parallel rays meet after re- 
flection called ? Why are burning mirrors made with a parabolic surface ? 




296 



CONCAVE REFLECTORS. 



Fig. 235. 



Diverging rays^ incident on spherical concave mirrors, will 
be converged to a point further from the mirror than the focus 
of parallel rays, or the principal focus. 

274. Images formed by concave mirrors vary in size and 
position according to the distance of the object from, the 
mirror, — Let o i, Fig. 235, be an object situated between the 

centre of curvature," C, of 
the mirror, and the prin- 
cipal focus, E. The rajs, 
7?i, G 71, from the point 
0, will, according to princi- 
ples already illustrated, be 
reflected, and intersect at 
0, to form an image of o 
behind C. The rajs from 
^ will in like manner form 
an image of it at I, and so ever j intermediate point between o i, 
will have its corresponding image between I. 

Thus, bj means of a concave mirror, m m', we maj form 
on a screen, placed bejond the centre of curvature, an inverted 
and enlarged image of the object, o i, Ijing between it and the 
principal focus. If, on the other hand, I be the object 
bejond the centre of curvature, the rajs from it meeting the 
mirror will form at o i, an inverted and smaller image. Bj 
placing the object nearer the mirror, between it and the prin- 
cipal focus, an image erect and greatlj magnified will appear 
behind the mirror. 

275. The above explanation of the properties of concave 
mirrors enables us to understand the manner in which manj 
optical phenomena, which have so astonished the ignorant of 




How will diverging rays incident on a concave mixTor be reflected ? What 
is said of images formed by concave miiTors ? Illustrate this by Fig. 235. 
If I be the object, where and what kind of image will be formed ? 
What wonderful phenomena does this explanation of the properties of the con- 
cave mirror enable us to understand ? 



COJ^VEX REFLECTORS. 297 

past ages, are produced. Thus a concave mirror concealed be- 
hind a partition may be made to throw a magnified image of 
an object, also concealed, through an open door, into an adjoin- 
ing room, upon a semi-transparent screen or cloud of vapor, 
causing it to appear suspended in the air before the eyes of the 
spectators. In this way, hideous images of skulls, daggers, etc., 
may be formed in the air without any visible cause. 

Concave mirrors are used as reflectors for light-houses, to 
render the light more intense in particular directions. These 
are also employed as hurn'mg mirrors. It was by means of 
an arrangement of reflectors, forming one huge concave mirror, 
that Archimedes was enabled to fire the Roman fleet under 
Marcellus, at a considerable distance from the walls of Syra- 
cuse. 

276. A convex mirror may be formed by polishing the 
exterior surface of a spherical concave mirror. 

Parallel rays of light falling on a convex mirror are 
Qnade to diverge as if proceeding from a point behind it. — 

This point is termed 
the imaginary or vir- 
tual focus. Let «, Z>, 
c, d, e. Fig. 236, be 
parallel rays, incident 
on a convex mirror, 
whose centre of cur- 
vature is C. These 
rays are reflected in 
the directions a', 6', d\ 
e', as though proceeding from a point, P, behind the mirror. 
277. The image formed by a convex mirror is erect and 



Where are concave mirrors used for aiding illumination ? What remark- 
able instance of their use as burning mirroi^s ? How may a convex mirror 
be formed? State the proposition in regard to parallel rays falling on a 
convex mirror. Explain this by Fig. 236. How do images formed by convex 
mirrors appear, as shown in Fig. 237 ? 




298 



REFRACTION OF LIGHT. 



onuch diminished in size. — Let B I, Fig. 237, be an object 
placed before the convex mirror, M M', whose virtual focus is 

Fig. 237. 




at E, and centre of convergency at C. The rays proceeding 
from B will be reflected from the convex surface to the eye at 
E', as though proceeding from a point, 6, behind the mirror ; in 
like manner those from I will appear to proceed from i, and so 
of all the intermediate points between B and I, thus presenting an 
image smaller, erect, and much nearer the mirror than the object. 



EEFRACTION OF LIGHT. 



Fig. 238. 




278. When light passes ob- 
liquely from one medium into 
another of difierent density, it 
suifers a deviation, or change 
of direction, known as refrac- 
tion. Eor instance, let a ray 
of light, proceeding from ?-', Fig. 
238, fall upon the surface of 
a body of water at S ; instead 
of continuing in a straight line 
to c?, it is turned aside at S, 



What direction does a ray of light take when passing from one medium 
into another of different density ? What is this change in the direction of the 
ray called ? Illustrate this by the passage of a ray of light from air into water, 
Fig. 238. 



REFRACTION OF LIGHT. 299 

and moves in the direction of S ?', more nearly in a line with 
P' P, the perpendicular to the surface of the liquid at the 
point, S. The angle, P' S r', Avhich the ray makes with the 
perpendicular before entering the water, is called the angle of 
incidence ; the angle, P S r, which it forms with this after 
entering it, the angle of refraction ; and d S r, the angle of 
deviation. Thus, whenever a ray of light passes from a rarer 
into a denser medium, it is bent toivards a perpendicular to 
that medium at the point of entry ; but, on the other hand, 
when it passes from a denser into a rarer, as from water into 
air, it is turned /rom this perpendicular. 

279. When a ray of light passes fro7n one mediwm into 
another of different density., the angles of incidence and 
refraction sustain to each other a constant ratio^ which is ex- 
pressed by their sines. — This may be illustrated by Fig. 238, 
where r S is the incident ray passing from air into water, and 
S r the refracted ray. Suppose a circle to be drawn, inter- 
secting these rays at r' and r ; the line r' ?^, drawn from 
the point of intersection, 7'', at right angles with the radius S 
P', will be the sine of the incident angle, and r m, drawn from 
the other point of intersection, at right angles with the radius S 
P, the si7ie of the refracting angle. These sines will vary with 
their respective angles, and of course with the obliquities of the 
incident and refracted rays to the perpendicular, P P'. 

Upon the passage of a ray of light from air into water, or 
other medium, these sines will be found to sustain to each other 
a uniform ratio. Thus, in the case of water, the sine of inci- 
dence is I ; that is, ?-' n is ^ of r m, the sine of the refracting 
angle. In the case of glass, | ; of the diamond, |, and so on. 
These fractions denote the refracting power of different media, 

In the figure, which is the angle of incidence ? Of refraction ? What is 
the course of light when passing from a rarer into a denser medium ? When 
a ray of light passes from one medium into another, of different density, what 
is said of its angles of incidence and refraction ? Explain this by the figure. 
What is the refracting angle for water ? For glass ? For the diamond. 



300 REFRACTION OF LIGHT. 

and are hence called their iiidices of refraction. Of the 
bodies mentioned, the refracting power of the diamond is the 
greatest, so turning the course of a ray of light incident upon 
it from a straight line towards the perpendicular, as to make 
its sine of refraction only f of the sine of incidence.* 

Inflammable bodies generally possess a much greater refrac- 
tive power than other substances of equal density; hence it 
was that Sir Isaac Newton was led to suggest the inflamma- 
bility of the diamond, long before it was shown by actual ex- 
periment to be inflammable. 

280. When the obliquity of an incident ray^ passi7ig 
through a denser medium towards a rarer ^ is such that the 
sine of its refracting angle is equal to radius^ it ceases 
to pass out, and is reflected from the surface of the 
denser medium, back into it again. — Let a pencil of 
diverging rays, proceeding from the luminous point, L, Fig. 
239, pass w^ith difierent obliquities towards the surface, S' 
S, of a body of water. The 
^^* * rays, L m, and L n, whose sines 

of refraction are less than radius, 
pass out of the water into the 
rarer medium, while L 0, and L 
P, angles of incidence greater 
than that at which the sine of the 
angle of refraction equals radius, 
are reflected back from the surface of the water, as from a 

* An amusing illustration of the bending of rays of light from a direct line, 
■when passing from a denser into a rarer medium, may be given by placing a 
cent in a bowl, so as to be just hidden from sight to a side observer. Pour 
■water into the bowl ; the coin will now become visible, and appear above its 
true position. From the same cause, ponds and rivers often appear to persons 
on their banks less deep than they really are. Such optical deceptions occa- 
sionally prove fatal to life. 

What is said of the refracting power of the diamond ? Newton's suggestion in 
regard to this? State the proposition, section 280, in regard to the reflection 
of light from the surface of a denser medium, as water. Explain this by Fig. 239. 




MIRAGE. 301 

perfect mirror. The angle at and within which this internal 
reflection occurs is called the limiting angle between refraction 
and reflection. 

This, for water, is 48° 28" ; for sulphur, 30°, and for the 
diamond, 23° 35'. Beyond this limiting angle, the reflection 
is total. Thus, if a wine-glass, nearly filled with water, be 
held up, so as that the surface may be seen from beneath, it 
will appear like a sheet of burnished silver. 

281. Mirage^ Fata Morgana^ and kindred remarkable atmos- 
pheric phenomena, are produced by the refraction and reflection of 
light, in its passage through, or incidence upon, strata of atmos- 
phere differing in density and refractive power, according to prin- 
ciples already explained. Thus, in certain states of the atmosphere, 
light passing from an object may proceed at such an obliquity 
as to be reflected from the upper surface of a denser stratum, 
and pass by refraction again to the earth. In such case, an 
object situated behind a hill, or below the horizon, may be 
brought to view, and appear suspended in the air, in an erect 
or inverted position. Such phenomena are often seen in great 
splendor in the Straits of Messina, on the deserts of Africa, 
and occasionally on the coasts of England and France, in the 
evenings of hot autumnal days.* 

282. Lenses. — These are certain forms of transparent 
bodies used for collecting or dispersing the rays of light which 

* Captain Scoresby and other voyagers in the polar seas relate seeing many 
remarkable instances of mirage ; as vessels actually below the horizon appear- 
ing moving under full sail, and inverted on the sky, etc. Travellers, in pass- 
ing across the heated sands of Africa, are often deceived by an appearance of 
■water in the distance. So villages far remote, and below the horizon, will at 
times appear painted on the sky, both in inverted and direct positions. These 
phenomena may be sometimes imitated by looking at objects over the surface 
of any heated body, as the boiler of a locomotive, when the objects will appear 
raised, or perhaps inverted. 

What is meant by the limiting angle ? What is said of the reflection of light 
beyond this limiting angle ? How may this be illustrated ? How is mirage 
produced ? Where is this often seen in great splendor ? What are lenses ? 

26 



302 



LENSES. — DOUBLE-CONVEX LENS. 



Fig. 240. 




pass through them. Fig. 240 presents sectional views of those 

commonly employ- 
ed for optical pur- 
poses. The double- 
convex lens, A, has 
two convex sur- 
faces ; the plane- 
convex^ B, has one 
convex, and one 
plane surface; the 
meniscus^ C, has a convex and a concave surface, the curvature 
of the former exceeding that of the latter, so as to produce a 
crescent form ; the double- concave^ D, has two concave surfaces ; 
the plane-concave^ E, a plane and a concave surface ; and the 
concave-convex^ F, a concave and convex surface, the curvature 
of the former exceeding that of the latter. The first three, 
which are thickest at the centre, are called convergent lenses, 
and serve to collect the rays of light passing through them to a 
focus ; while the last three are termed divergent lenses, and 
act to separate these rays. 

283. Parallel rays incident on a double-convex lens are 
converged to a focus at a distance from the lens, varying with 
the curvature of its sides. — Let a, b, c, d, e. Fig. 241, be 
parallel rays incident on the double-convex lens, L' L. In 

passing through 
the lens, these 
rays will under- 
go refraction, and 
be converged to 
a point, F', on 
the axis, known 
as the common 




What kinds of lenses are shown by Fig. 240, and how are they formed ? 
How do the first three lenses differ from the last three ? What is said of par- 
allel rays incident on a double-convex lens? Explain this by Fig. 24L 



DOUBLE-CONCAVE LENS. 



303 



focus. With lenses of the same refracting medium this focus 
will vary according to the curvature of the sides. 

If the rays falling on the lens, L' L, be converging^ their 
focus will be nearer the surface of the lens than F'. If, on the 
other hand, they be divergent^ as from the point, r, they will 
have their focus at a point, F, farther from the lens, than that 
for parallel rays. 

284. The rays of light traversing a double-convex lens are 
not all converged to the same point on the axis. This defect is 
remedied in a good degree by having the lens of a parabolic 
form. In the common form of the lens, those rays which pass 
through it near its edges are converged to a focus nearer the 
surface of the lens than those which traverse it more nearly 
parallel with the axis. Such varying of the focal points for 
rays traversing the lens at different distances from the axis, is 
termed the spherical aberration of the lens. To avoid this 
aberration in lenses employed for optical purposes, a meniscus 
(Fig. 240, C) is used with the double-convex lens, by which 
the rays are made to converge uniformly to a single point. 

285. Rays falling on a double-concave lens are rendered 
more divergent after than before passing through the lens. 
— The parallel rays, a, 6, c, d^ e, Fig. 242, falling on the 

double-concave lens, 

Fig. 242. 




L L', after 



passmg 



through this, are 
made to diverge as 
though proceeding 
from the point, c. So 
convergent rays are 
rendered less conver- 
gent, or even parallel. 



How will be the focus of diverging and converging rays incident on V L ? 
What is said of the rays traversing a double-convex lens ? How is the aber- 
ration of light in such cases remedied in good part ? The course of rays of 
light traversing a double-ooncave lens ? Explain Fig. 242. 



304 



IMAGES FORMED BY LENSES. 



1 



Tims, the refraction of light by concave lenses corresponds 
to its reflection bj convex mirrors, both being dispersive in 
their effects. So, also, the convex lens and concave mirror 
correspond in their converging power over light. 

286. Images are formed by lenses in the same m^anner as 
by mirrors. — Let 0', Fig. 243, be an object placed before 



Fig. 243. 




the lens, L L', just without its principal focus, F. The rays 
proceeding from the point, 0, will be converged by the lens, 
and form an image at o ; those from 0' will also be converged, 
and form an image at o' ; and so each point between 0' 
will have its corresponding image between o o'. Thus, an inverted 
and magnified image of the object will be formed on a screen 
at 0, o', owing to the crossing of the rays, and their divergence 
from the point, F. 

If the object, 0', be brought nearer L L', the image, o 
o', will proportionably recede, and become magnified, so that 
the eye, placed at a favorable point without F, will see an 
image of the object magnified in proportion to its nearness to 
the point, F'. 

To what does the refraction of light by concave and convex lenses corre- 
spond ? What is said of images formed by lenses ? Explain the manner in which 
images are formed by a double-convex lens, as sliown by Fig. 2-43. How will 
the nearness of the object to the lens affect its apparent size? 



DECOMPOSITION OF LIGHT. 



305 



If, in place of the double-convex, a double-concave lens be 
employed, the image will appear smaller than the object. 

This wonderful property of lenses depends upon the appar- 
ent angle under which the object is viewed, the eye' seeing the 
object in the direction in which the rays from the object enter 
it ; so that if these rays be converged to it at a large angle, as 
in case of the magnifying lens, the object from which they 
proceed will appear to span the same angle. Hence, the 
shorter the focal distance of a lens, the greater will be its mag- 
nifying power, and vice versa. 

DECOMPOSITION OF LIGHT. 
287. White solar light is a compound formed by the blend- 
ing of different colored rays. — This may be proved by passing 
a ray of sunlight through some highly refracting medium, 
whereby the colored rays composing this, and which have 
different degrees of refrangibility, shall be separated. The in- 
strument commonly employed for this purpose is a triangular 
prism of flint glass, with its three polished sides usually in- 
clined to each other at angles of 60°. 

Experiment. — Through a small opening, 0, Fig. 244, 

allow a ray of light to 
enter a dark room, and 
fall on a white wall, or 
screen at W, where will 
be seen a small round 
spot of white light. In- 
terpose now a prism, 
P, and the ray, instead 
of passing in a direct 
line to W, will be re- 
fracted to S S, forming 

If a double-concave lens be employed, how will the image appear ? Upon 
what does this wonderful property of lenses depend ? What is white solar 
light ? How may this be proved ? What is a prism ? Explain the manner 
in which a beam of light may be decomposed, as seen in Fig. 244. 

26* 



Fig. 244. 




306 DECOMPOSITION OF LIGHT. 

between those points an elongated spectrum^ composed of bands 
of seven different colors insensibly passing into each other. 
These seven colors, of which the solar ray is composed, namely, 
red, orange, yellow, green, blue, indigo, and violet, wall appear 
in the spectrum in the order of their initials ; red being the 
least refracted and low^est, and violet the most refracted and 
highest, in the series of colors. 

Herschel, in his Treatise on Light, thus describes these colors 
formed by the decomposition of a solar ray. '' On viewing the 
spectrum attentively, we perceive that the lowest or least re- 
fracted extremity is a brilliant red, more full and vivid than 
can be produced by any other means, or than the color of any 
natural substance. This dies aw^ay, first into an orange, and 
then passes, by imperceptible gradations, into a fine pale straw 
yellow, which is quickly succeeded by a pure and very intense 
green, which again passes into a blue, at first of less purity, 
being mixed with green, but afterwards, as we trace it upwards, 
deepening into the purest indigo. Meanwhile, the intensity of 
the illumination is diminishing, and in the upper portions of the 
indigo tint it is very feeble, but is still continued beyond, and 
the blue acquires a pallid cast of purplish red, a livid hue 
belter seen than described, and which, though not to be ex- 
actly matched by any natural color, approaches most nearly to 
that of a fading violet." 

If these seven colors of the solar spectrum be separately 
submitted to the action of a second prism, they will be re- 
fracted, but undergo no further change of color ; thus showing 
them to be simple or primary colors.* 

* Some ingenious experiments by the eminent optician, Dr. Brewster, go far 
towards proving the existence of only three primary colors, namely, red, 
yellow, and blue ; the other four coloi'S of the solar spectrum being formed by 
an intermingling of these three. 

Oi'der of the colors of the solar spectrum ? Which color of the spectrum is 
refracted most, and which the least ? Describe these different colors. If these 
seven colors be separately submitted to the action of a second prism, can they 
be iurther decomposed ? 



DECOMPOSITION OF LIGHT. 307 

288. By re'uniting these seven jjrimary colors of the 
spectrum^ v;hlte or solar light may he jyroduced. — This may 
be effected bj an arrangement seen in Fig. 245. Let the 

various colors of a 

^'^' ^^^' solar ray, decomposed 

^^^^^^^^^^I^^^^^^H by the prism, P, fall 

^^^^^^K^^^M^KBK^^^ 0^ the double-convex 

H^^^^^^^^^^^^^^^Sb lens, V r, by which 

IQ^^^^^^^^^^HHj^H they will be converged 

^^^^^H^^^^^^^^^^HI made to 

^^^^^^^^^^^^^^^^H be 

^^^^^^^^^^^^^^^^^Hb point of solar 

BH^BbIJUUB^^BI light will be seen. 

Upon removing the 
screen to r v, the colors of the spectrum will be again seen in 
an inverted order, from that formed at v r. A concave mirror 
may be substituted for the lens, and the colored rays converged 
to a point with the same result. 

By dividing a circular disk into seven sectors, and painting 
these with colors most nearly approaching the prismatic hues, 
and then causing this disk to revolve rapidly, as in § 246, in- 
stead of either color there will be visible only a grayish white, 
caused by the blending of the seven primary colors on the 
retina of the eye. The impossibility of obtaining perfect im- 
itations of the prismatic colors, renders the mingling of arti- 
ficial spectra an impure white. 

289. Achromatic* Lenses. — Light, in its passage through 
different substances, undergoes different degrees of dispersion, 

* u, without, /QoHia, color. 

Result of reuniting these prismatic colors ? How may this reiinion be ef- 
fected, as shown by Fig. 245 ? If a disc on which are painted the seven 
primary colors be rapidly revolved, what will be the result? Why cannot a 
pure white be obtained in this case ? What is said of the dispersion of light 
in its passage thi'ough different substances ? 



308 ACHROMATIC LENSES. 

or separation into its elementary colors. Thus, a ray traversing 
a prism oi Jluit-glaiSS will have its red and violet colors separated 
on a screen tivice as widely as those of a ray passing through a 
similar prism of a^oum-glasSj while the refracting power of 
both glasses are very nearly equal. Thus we say that the 
decomposing or dispersive power of flint-glass is twice as great 
as that of crown-glass. 

Every simple lens, of whatever substance made, w^ill have 
a different focus for every different color of which a solar ray 
is composed ; the focus of the red ray lying further from the 
lens than that of the violet. It is from this cause that the 
images of such lenses appear more or less impure, or bordered 
wdth colored edges, when we look through them at the print 
of a book, for instance. 

Lenses which refract light, without at the same time decom- 
posing it, are termed achromatic lenses. To prepare such was 
for many years the desideratiim of opticians. Achromatic 
lenses were at length obtained by combining lenses made of 
different kinds of glass. Such a combination is seen in Fig. 
246, where a convex lens of crown-glass is 
^'g- ^'^^- united with a concave lens of flint-glass so 

as to destroy each the dispersive power of 
the other, and produce no dispersion at all, 
while at the same time the converging power 
cf the convex lens is preserved. Thus, the 
light passing through these suffers conver- 
gence without dispersion. 
290. The Rainbow is a result of the decomposition of the 
solar rays by the drops of rain, and the separation of these 
rays into the elementai*y colors of which they are composed. 
This consists of a brilliant-colored arch spanning the heavens 

Illustrate this by prisms of flint and crown glass. "Will a single lens con- 
verge all the primary colors of a solar ray to the same focus ? What are 
Achromatic Lenses ? How may such lenses be made as seen in Fig. 246 ? How 
is tlie Rainbow caused ? Of what does this consist, and where formed ? 




THE RAINBOW. 



309 



opposite the sun, and usually has a second less brilliant attend- 
ant just above it, termed the secondary bow. The rainbow is 
only seen when a shower of rain is falling, or the spray from a 
cataract is rising between the spectator and that portion of the 
heavens opposite the sun. 

The cause of this phenomenon may be briefly stated as fol- 
lows : Imagine a straight line passing from the sun through 
the eye at E, Fig. 247, and proceeding to the centre of the 

Fig. 247. 




bow. Let r v' be two drops of rain, on which the solar rays, 
S S, are incident. The ray falling on the upper portion of the 
drop will be refracted, and, passing to the back of it at an 
angle with the surface of the drop at that point, within the 
limiting angle (§ 280), will be totally reflected and emerge 
at the lower side, where it will again undergo refraction, and 
pass to the eye at E. 

Thus, to produce the primary bow, light comes to the eye 



Explain the phenomenon of the primary bow as shown by Fig 
give the course of the rays through each drop. 



247, and 



310 POLARIZATION OF LIGHT. 

after undergoing decomposition and suffering two refractions 
and one reflection ; each drop, in a vertical series, transmitting 
to the eye a particular color according to its position and the 
angle -which it makes with the imaginary line passing from the 
sun through the eye to the centre of the circle ; thus forming 
a bow of prismatic colors around this line or axis, bounded 
upon the upper side by the red, and upon the under by the 
violet rays (§ 287). 

When the solar rays enter the rain-drops from beneath, as 
at V r in the figure, they are refracted to the back of these 
drops as before, but, instead of a single reflection, they suffer 
tico reflections^ and emerge from the upper side of the drops, 
presenting to the eye a second bow exterior to the first. The 
order of the colors of this secondary bow will be reversed and 
much fainter than in the primary, owing to the rays having 
undergone two reflections instead of one ; each reflection and 
refraction serving to disperse or absorb a portion of the light, 
and so render the colors less brilliant than in the primary or 
inner bow. 

291. Polarization of Light is that peculiar change which 
light undergoes when reflected from certain surfaces at particu- 
lar angles, or when transmitted through certain crystals, as 
Iceland spar. 

If, for example, rays of light fall on a glass plate blackened 
on its back, so that its surface shall make with these an angle of 
54° 35', these rays will be reflected according to the usual laws. 
But if these reflected rays fall on a second and similar glass 
plate so as to make with its surface also an angle of 54° 35', they 
will be reflected from this second surface only in certam 
positions of the plate. Thus, if this second plate be revolved so as 
to keep its parallelism at two points in its revolution, it will 

Explain the course of the rays of light in producing the secondary or outer 
bow. Why are the colors of this secondary bow less brilliant than those 
of the primary? What is meant by Polarization of Light? Illustrate this in 
the case of two glass plates blackened at their backs. 



CALORIFIC EAYS. 311 

reflect the light from the first plate as usual, while at two 
other points this reflection will wholly cease. In this case the 
rays by reflection from the first plate undergo a peculiar modi- 
fication, whereby they deviate from ordinary light in respect 
to their laws of reflection. Such rays of light are said to be 
polarized. 

292. If we place a crystal of Iceland Spar on the letters of 
a book, for instance, they will appear double^ owing to the rays 
of light in their passage through the crystal having undergone 
a double refraction; so of other objects viewed through it. 
If we examine, through a plate of tourmaline^ the two images 
seen through the Iceland Spar, we shall find that both rays 
are polarized ; for, as we turn the tourmaline plate, these rays 
will be transmitted through it only in certain positions of 
its revolution; the images alternately appearing and disap- 
pearing. 

293. Calorific Rays. — Light and heat are separate and inde- 
pendent agents. This may be shown by placing a highly sen- 
sitive thermometer in the different rays of the solar spectrum 
(^§ 287), when it will be found that the yellow ray, which is the 
most luminous, is far from being the hottest ray of the solar 
spectrum, while the red ray, which yields comparatively little 
light, produces a degree of heat exceeding that of any of the 
other primary rays. 

If the thermometer be carried a little below, and just out 
of the red ray, into the darkened space, it will show a consid- 
erable increase of heat, thus proving the presence of a heat- 
ing ray in solar light, independent of the luminous principle. 

294. Chemical Action of Solar Light. — The effect of solar 
light, in producing chemical changes in certain substances, has 
been known for ages ; but not until within a comparatively 
recent period were the nature of these changes, and the pre- 
cise manner in which they are effected, known. 

What is said of the effects of Iceland Spar on light traversing it ? Are light 
and heat independent agents ? How shown to be such ? 



312 CHEMICAL ACTIO:^ OF LIGHT. 

'Chlorine and hydrogen gases may be mixed together, and 
kept in a dark i^oom^ for any length of time, without uniting or 
suffering the least change ; but if, when thus mixed, they 
be exposed to the clear light of the sun, they will be made to 
unite rapidly and produce an explosion, when a strong pungent 
acid (hydrochloric acid) will result. Solar light also promotes 
the union of the oxygen of air with the carbon and hydrogen 
of organic substances : hence the darker hue and substantial 
character of vegetables reared in sunlight compared with those 
grown in the shade. 

The chemical effects of solar light are most remarkably seen 
in its action on certain salts of silver. Thus, paper covered 
with a thin coating of chloride of silver — a white salt — rap- 
idly changes its color, and becomes blackened when exposed to 
sunshine ; and if any opaque body, as a leaf, or piece of figured 
lace, be placed upon this paper before exposing it, the portions 
of the paper where the sun's light is intercepted will remain 
unchanged, leaving an exact copy of the object. Upon remov- 
ing the object, the picture rapidly changes, and the whole sur- 
face of the paper becomes uniformly black. 

Such impressions, made on paper suitably prepared, and then 
by certain subsequent appliances rendered permanent, consti- 
tutes the Photograph or Calotype art.* 

* We have been kindly furnished with the following modern process for 
taking photographs and daguerreotypes, by Mr. John A. Whipple, of this 
city. Mr. "Whipple is an artist who has acquired a distinguished reputation 
by the extraordinary success which has attended his efforts as a photographer 
and daguerreotypist. 

Photographs are taken either on glass, or on paper of the finest quality and 
most compact material. The compound spread upon these to form a surface 
sensitive to light is prepared as follows : Collodion, a substance prepared by 
dissolving gun-cotton in alcohol and ether (six parts alcohol and eight of 
ether), is mixed with bromine and iodine in the proportion of three grains of 
iodine and two of bromine to about an ounce of collodion ; this compound is then 
spread evenly on the surface of the glass or paper ; the surface, thus prepared. 

Chemical effects of solar light, how shown ? What surftices are particularly 
sensitive to this ? In what does the photographic art consist ? 



THE DAGUERREOTYPE. 313 

295. TJie Daguerreotype. — This is a process by which 
solar light is made to paint the images of objects on highly pol- 
ished metallic surfaces, instead of on paper or glass, as in case 
of the calotype just referred to. 

This method of taking pictures is also due for its success to 
the chemical action of light on surfaces rendered sensitive by a 
delicate covering of iodide, cyanuret of silver, or some other 
substance easily changed by the presence of sunlight. 

When such a surface, properly prepared, is suddenly exposed 
in the camera obscura^ it has speedily impressed on it an 
exact outline image of the object or objects before it ; the 
light from the darker portions of the object, acting less upon 
the sensitive surface, leaves a negative picture of those portions, 
while that from the lighter portions, or from the spaces where 
it is not interrupted, produces a further action, and leaves a 
darker or positive impression. Thus, portraits, artificial views, 
landscapes, etc., are sketched by the solar rays with a pre- 
cision and accuracy far exceeding that from the pencil of the 
most skilful artist. 

In this process, the scene presents only a light and a shade. 
Efibrts have, however, been made to paint and fix the natural 
colors of objects sketched in the camera. This has been effected 
in part, and is known as the Hillotype process, from the name 
of the discoverer, Mr. Hill. At present these colors are made 
to appear upon the paper or plate, but gradually fade away and 
disappear under the continued action of light and air. That 

is then immersed in a bath containing a solution of nitrate of silver (thirty 
grains nitrate of silver to one ounce water), where it remains until it acquires 
a brownish color, or until the greasy appearance is removed, when it is care- 
fully screened from the light, and subsequently placed in the camera-box. 

When the person or other object of which a picture is to be taken is ar- 
ranged at the proper distance, and in a desirable position for forming a suit- 
able image, the slide or screen of the camera-box is suddenly raised, and the 
light from the object allowed to ftill on the sensitive surface. An impercepti- 
ble image will be formed in from one-tenth of a second to three minutes, the 

Describe the Daguerreotype process. What is the Hillotype ? 

27 



314 THE DAGUERREOTYPE. 

the efforts now made by the French artists as well as those of 
our own country, will soon prove successful in rendering per- 
manent the natural colors of objects painted in the camera, we 
cannot doubt. 

Thus, when the delicate expressions of the countenance, the 
minute and varied outline and changing hues of the landscape, 
shall be sketched and painted by that most perfect of artists, 
the sun ; when thought shall speed its way from continent to 
continent, borne with the timeless flight of the lightning, then 
may be realized in some good degree the full force of the 
maxim, '' Truth is stranger than fiction." 

time varying with the degree of light and sensitiveness of the surface ; the 
picture is then developed by pouring over the surface of the glass or paper a 
solution of sulphate of iron (copperas), of the strength of 8 or 10 Baume's 
hydrometer, mixed with about one-third its bulk of acetic acid. To fix the 
picture, the surface is finally washed with a solution of hyposulphate soda, 
when, after drying thoroughly, it is ready for the frame or case. 

Daguerreotypes may be taken by the following process. Smooth and even 
plates of sheet copper, cut of the proper form and size, are silvered by galvan- 
ism. (See § 205.) This coating of silver is then scoured with a mixture of 
ammonia, alcohol and rotten-stone, and afterwards receives a bright polish on 
a bufl:-wheel by means of rouge. It is then dipped in a solution of cyanuret 
of silver, and subjected for two or three minutes to the galvanic process, when 
it receives an exceedingly delicate coating of silver ; after rinsing ofi" the resi- 
due with pure water, and drying over a spirit lamp, it receives a second polish 
on a buff-wheel. This surface is now placed over a vessel, upon which it fits, 
and is exposed to the vapor of iodine until it assumes a color between that of 
lemon and orange ; it is then placed over a second vessel containing bromide 
of lime, heated to 212° F., until the surface acquires a pink color, after which 
it is again exposed to the vapor of iodine for about one-third the time of the 
first exposure to this. The plate is now ready for the camera-box, in whicli, 
screened from light, it is placed, and the picture taken, as in the photograph 
process just described. No image appears, however, until the plate has been 
placed over the vapor of mercury, heated to 212^ F., in an iron box, — the 
time of its exposure to this vapor varying from one-half to three minutes ; 
if exposed too long, the picture will be light and faint ; if for too short a time, 
it will appear too dark. The process of fixing the picture is the same as witli 
the photograph. 

What is said of the probability of being able to fix the colors tlius taken ? 



STRUCTURE OF THE EYE, 



315 



THE EYE. 



296. The eye is that organ of sense which conveys to the 
mind an image of the external world. Its superior importance 
may be understood by considering the vast amount of ideas it 
presents to the mind. By it we are enabled to judge with 
great readiness and precision of the magnitudes, forms, motions, 
distances, and positions, of the endless variety of objects which 
come within its scope. The eye is the most perfect of all 
optical instruments, and a knowledge of its structure is neces- 
sary for comprehending those artificial structures which the 
ingenuity of man has devised as aids to vision. 

297. The Structure of the Eye. — The human eye is of a 
form approaching a sphere, and is placed in a bony cavity at 
the side of the upper portion of the nose. This consists of an 
assemblage of lenses so arranged as to concentrate the light 
from each point of external objects on a delicate tissue of 
nerves called the retina, there forming an image or exact 
representation of the various objects perceived by the mind. 
Fig. 248 presents an enlarged sectional view of the different 
parts which compose the human eye. The outside covering, 

S S S S, is the sclerotic 
coat^ a tough white mem- 
brane commonly known as 
the IV hit e of the eye. In 
the front of this sclerotic 
coat is a circular transpa- 
rent opening, having a clear, 
horny covering, c c, pro- 
jecting somewhat beyond 
the other portions of the 
eyeball. This is called 



Fig. 248. 




What is said of the Eye, and what does it convey to the mind ? What is 
said of it as an optical instrument ? Where is it placed ? Of what does it 
consist ? What is the retina ? What is the sclerotic coat, and how situated ? 



316 STRUCTURE OF THE EYE. 

the cor7iea^ and incloses on one side a small chamber filled 
TYith a transparent liquid, known as the aqueous humor. The 
form and consistency of this aqueous humor is just what is 
required for aiding in converging the rays passing through 
it to a focus at the precise point necessary for distinct vision. 

Within this chamber, and partly dividing it, is the iris^ 1 1, a 
circular opaque screen or diaphragm, with a round and appar- 
ently dark opening in its centre, called the inipil. It is the 
iris which determines " the color of the eye," as gray, blue, 
or black, and, by its contraction and expansion, increases or 
diminishes the size of the pupil, and so regulates the quantity 
of light admitted to the retina. In the posterior part of the 
chamber containing the aqueous humor, is the crystalline lens^ 
Z, which is a double- convex lens, exhibiting in its form and 
composition a wonderful contrivance for preventing that spher- 
ical aberration, to which convex lenses are usually subject. 

The cavity, v v v v^ behind the crystalline lens, is occupied 
by the vitreous humor ^ a transparent gelatinous fluid, muck 
resembling the white of an egg. Lining the inner surface of 
the sclerotic coat, is a dark membraneous substance, a a, called 
the choroid covering^ which serves to absorb the light as soon 
as it has acted on the retina, and thus prevents internal reflec- 
tions, and consequent confusion of vision. 

The retina is a delicate network of nerves spreading over 
the choroid surface, and appears to be only an expansion of the 
optic nerve, n. It is upon this that the images of external 
objects are cast, of which an impression is conveyed by the 
optic nerve to the brain. 

298. The images of external objects are inverted on the 
retina. — The refracting mediums of the eye act on the rays 

The cornea? The aqueous humor ? What is said of the aqueous humor? 
What is the iris? What is the crystalline lens, and where situated? What is 
the vitreous humor ? What is the choroid covering, and its use ? Describe the 
retina. How are the images of external objects formed on the retina ? 



VISION. 317 

passing through them, similar to a convex lens (§283), causing 
an inverted image of objects to be formed on the retina. Tig. 
249 will serve to show the course of light proceeding from an 
object to the eye. These rays are seen to be converged by the 
humors or lenses of the eye, and cross just behind the crystal- 
line lens, so as to form a minute inverted image on the retina. 

This may be shown by taking the eye of an ox, or other 
large animal recently killed, and removing the posterior portions 
so as to lay bare the choroid membrane. If the eye, thus pre- 
pared, be fixed in a screen, and a lighted candle be placed before 
it, at the distance of fifteen or twenty inches, a minute inverted 
image of the candle will be seen through the retina, as if pro- 
duced by a double-convex lens on a screen of gi'ound glass, or 
oiled paper. 

299. The eye possesses the remarkable power of adapt- 
iiig itself to objects at varying distances. — We have already 
seen that the focus for light of a convex lens is constantly 
changing with the distance of the object and the angle which 
the rays proceeding from it make when incident on the lens. 
Hence, to preserve the focus at the same distance from the lens, 
it is necessary either to vary its form and power of refraction, 
or its distance from the object. 

In the eye, this uniformity of the focal distance is most accu- 
rately preserved by an involuntary change in the convexity 
of the refracting mediums, thus varying the form of these me- 
diums with the angle of the incident rays, and so causing the 
focus of the light to fall exactly at that point necessary for 
forming a perfect image on the retina. This power of the 
eye to adapt itself to objects situated at different distances from 
it, may be proved by the following 

Experiment. — Let a small black spot be made on a thin 
transparent plate of glass, placed about twelve inches from the 

How may the course of light through the eye and the images formed on the 
retina be shown ? What is said of the power of the eye for adapting itself to 
objects at varying distances ? Give the experiment illustrating this. 
21* 



318 



VISION. 



eye. If the eye be directed to it, the spot will be seen, as well 
as distant objects visible through the glass. Let the attention 
be earnestly directed to the black spot, so that a distinct percep- 
tion of its form may be produced. The objects visible at a 
distance will then be found to become indistinct. But if the 
attention be directed more to the distant objects, so as to ob- 
tain a distinct perception of them, the perception of the black 
spot on the glass will become indistinct.* 

300. The appaient size of an object depends on the size 
of the visual angle under wJiicJi it appears. — Let A B, 
A' B', A" B", Fig. 249, be three objects differing in their 

Fig. 249. 




vertical heights, in proportion to their distances from the eye. 
As each subtends the same visual angle, their apparent heights 
will be equal. Thus, a small gnat near the eye may cover the 
same angle and appear equal in size to an eagle at a distance. 

301. TJie eye stipplies no direct perception of the mag- 
nitude and distance of objects ; these being determined by 
an exercise of the judgment based on experience. — Thus, 
we judge of the height and distance of a church-steeple, or 

* Lardner. 



Upon what does the apparent size of an object depend ? Hoav does Fig. 249 
show this ? What is said of the eye in reference to the magnitude and dis- 
tance of objects? How do we judge of the height and distance of any object, 
as a church-steeple for instance ? 



DEFECTS OF VISION. 319 

tower, by comparing these with known objects which intervene. 
When no such objects stand between the eye and the distant 
body, whereby a comparison may be made, we often err greatly 
in our estimates of these. This is especially true in viewing 
objects on the ocean.* Hence it is, also, that the sun and 
moon appear larger when rising and setting, than when at the 
zenith ; these, at such positions, being compared with the inter- 
vening objects, whereby a false estimate of their visual dimen- 
sions is obtained. 

On the contrary, we often judge an object to be much smaller 
than the reality, when viewed alongside some body of surpassing 
dimensions. Thus, a first-class merchant-man may be esti- 
mated no larger than a small barque when seen moored beside 
a three-decker ship-of-the-line ; or dwellings of ordinary size, 
viewed in contrast with St. Peter's church at Rome, or the 
Capitol at Washington, may seem like mere cottages. 

302. Near and far sightedness are occasioned by too 
great or too slight convexity of the refracting medium of 
the ez/e, whereby rays of light are brought to a focus before or 
behind the retina. — When the convexity of the cornea is too 
great, as in cases of near-sightedness, the eye has not the power 
of diminishing this convexity, sufficient to throw the focus of 
lays from objects making a small angle with it back upon the 
retina. In such case only a confused image is formed on the 
^ retina. To prevent this, it is necessary to increase the angle 
which the rays make upon entering the eye, either by a use of 

* The dim perception of vessels seen through a fog often deceiyes the judg- 
ment in regard to their size, since the appearance, through such a medium, 
le;xds us to suppose them at a much greater distance than they really are. 
Thus, an ordinary sail-boat, under such circumstances, has been often mistaken 
for a sloop or schooner ; these, in such cases, are said by sailors to loom up. 

Where is our judgment in regard to distance and magnitude especially lia- 
ble to err? What is said of our judgment of the size of objects in contrast 
with those much larger ? How are near and far sightedness occasioned ? In 
case of near sight, how is the light converged by the crystalKne lens? How 
may this be prevented ? 



320 VISION. 

small double-concave lenses (<§> 285), called spectacles^ or by 
bringing the object nearer the eye. 

On the other hand, when the convexity of the cornea becomes 
too much diminished, as in old age, the rays are not converged 
to a point sufficiently soon, but have their focus behind the 
retina. To remedy this defect by increasing the convergence 
of the light behind the crystalline lens the angle of the rays 
incident on the lenses of the eye must be diminished; this 
may be effected, either by. holding the object, as a book, for 
instance, at a distance from the eye, or by the use of spectacles 
■with convex lenses.* 

303. The i7npression of a visible object on the retina 
lasts for an appreciable time after the object is removed. — 
If a fire-brand be made to revolve rapidly before the eye, an 
entire circle of light will be seen ; for the impression made on 
the retina, by the light at any point of the cii'cle, remains until 
the brand returns to that point again. f So, also, of lightning 

* Defective sigM may arise from various causes. Thus, near-sightedness may- 
be caused by a too great convexity of the cornea or the crystalline lens, or from 
too great a difference of density between the aqueous and the crystalline humors, 
or between the crystalline and the vitreous humors, or both of them ; or it may 
be ijaused by defects both of the form and the relative densities of the humors. 
Sight is sometimes injured or well-nigh destroyed by the crystalline humor 
losing its transparency in a greater or less degree, and thus preventing the light 
from reaching the retina, or from reaching it in a proper state to form an image. 
This is seen in cases of cataract. Such a defect may be often remedied by re- 
moving the crystalline humor, and leaving the light to be converged by the 
aqueous and vitreous humors only. If these be not sufficient to converge the 
light, they may be assisted by convex spectacles. 

t The impression of light on the retina lasts from one-seventh to one-tenth 
of a second, varying according to the vividness of the light producing the im- 
pression. The state of the illumination of the surrounding space also varies 
the time of the impression. A luminous object in a dark room produces an 
impression more lasting than the same object in a light room. This is prob- 
ably due to the greater sensitiveness of the retina when in a state of repose, 
than when its entire surface is excited by surrounding lights. 

How is defective sight in old age produced, and how may this be remedied? 
What is said of the impressions on the retina after an object is removed ? 
Why does a fire-brand whirled in the air produce a circle of light ? 



PHANTASMASCOPE. 321 

and meteors which exhibit long luminous lines ; the impression 
of the light from the first part of their course not being re- 
moved before that from the last is received. 

The Phantasmascope^ an optical toy, acts on this principle. 
This consists of disks, bearing on their margin a variety of 
figures, which are so related to each other, that each succeeding 
figure shall afibrd a continuation of the preceding, and the 
whole taken together, when put in rapid revolution, shall ex- 
hibit a single figure, performing some singular or amusing feat. 
Thus, the figure might commence with a player, holding a violin 
and a bow which is just beginning to draw ; the second view 
might represent the bow as drawn a little; the third, still more; 
and the whole views would then exhibit the usual motions of 
the bow. In the same manner are performed dances, feats of 
horsemanship, and the like.* 

304. Spectral Colors. — When the eye has gazed fixedly 
for some time on an object of a particular color, strongly illu- 
minated, upon turning suddenly to a dark or white wall, it will 
continue to see an image of the object, but of a color quite 
different from the original. This image is called an ocular 
spectrum^ and the colors it presents accidental colors. Thus, 
if a bright-red figure painted on a dark surface be intently 
watched for a few moments, and then the eye turn to a white 
wall, the same image will continue and appear on the wall, but, 
instead of red, it will appear of a bluish-green color, or that 
color complementary to the original color of the object. So, if 
it be yellow, the spectra will appear of a deep violet. 

These complementary colors, at first quite distinct, gradually 
fade, passing into others, until the spectral image vanishes. This 
phenomenon is occasioned by the eye becoming partially para- 

* Olmsted's Philosophy. 

Describe the operation of the Phantasmascope. When are ocular spectra pro- 
duced ? What are these colors produced called ? What relation do the colors 
seen under these circumstances sustain to each other ? Illustrate this. How 
is this phenomenon occasioned ? 



322 DEFECTS OF VISION. 

lyzed by the dazzling effect of the light from the object, and so 
indifferent to the particular tints it presents, but more sensitive 
to those colors most widely opposed to these. It is, therefore, 
that the eye, in the case of the red figure, perceives its comple- 
mentary color, green, and continues to perceive it until the 
retina again recovers its sensibility for red light. 

305. Defect of vision from, an inability to distinguish cer- 
tain colors. — Persons are occasionally met with, whose 
vision, although sound in all other respects, is singularly de- 
fective in the power of distinguishing particular colors of the 
spectrum. 

Sir David Brewster relates the case of a shoemaker, by the 
name of Harris, who always mistook orange for grass-green, 
and light-green for yellow, and from infancy was unable to dis- 
tinguish the cherries of a cherry-tree from its leaves, so far as 
color was concerned. 

Another amusing instance is given of a tailor, who con- 
founded green with red, and who one day, by mistake, repaired 
a coat, of dark-green color, with a piece of cloth of scarlet 
color. 

Such states of vision are attributed by Sir J. Herschel to a 
defect in the sensorium, by which it is rendered incapable of 
appreciating exactly those differences between rays on which 
their color depenJs. 



OPTICAL INSTRUMENTS. 

306. The Microscope.* — This instrument, designed for aiding 
the eye in perceiving minute objects, may be regarded as among 

* JSliy.Qoq, small, oy.omia, to see. 



What is said of defects in the vision of some persons in regard to their abil- 
ity to distinguish colors ? Case of the shoemaker ? The tailor ? How does 
Sir J. Herschel explain the cause of these singular defects ? For what is the 
Microscope designed ? 



SIMPLE MICROSCOPE. 



823 



the most remarkable achievements of modern art, since its dis- 
covery has brought to view a new world of being, and disclosed 
processes in nature equally wonderful and important. 

We have already seen (§300) that the apparent size of 
objects depends on the angle under which they are viewed, 
and of course upon their nearness to the eye. When, how- 
ever, an object approaching the eye reaches a certain point, 
its rays meet this at angles, too diverging to be collected by 
the crystalline lens on the retina, and vision begins to grow im- 
perfect. This point is known as the limit of distinct vision^ 
and is usually dhoMiJive inches distant from the eye. 

Every instrument, therefore, which will admit of viewing 
objects nearer than this limiting angle, may be regarded as a 
microscope. 

307. A Simple Microscope is merely a convex lens, of 

short focal distance, by 
which the rays from near 
objects may be converged to 
the eye, so as to render these 
visible. The magnifying 
principle of this microscope 
may be illustrated by Fig. 
250. 

308. Let 0', Fig. 250, 
be a minute object placed 
before the double-convex lens, F F', at or just without its prin- 
cipal focus, but too near the eye to be seen by it without arti- 
ficial aid. The rays which fall on the lens from 0', and 
every intermediate point of the object, will be converged, and 
enter the eye, at E, causing an image of the object, greatly 



Fig. 250. 




How may this instrument be regarded ? Why cannot minute objects near the 
eye be distinctly perceived ? What is meant by the limit of distinct vision, and 
how far is this usually from the eye ? AVhat use does the microscope serve ? 
What is a Simple Microscope? Explain the manner in which the microscope, 
as seen in Fig. 250, enables the eye to distinguish minute objects very near 
it, by magnifying their apparent dimensions. 



324 



COMPOUND MICROSCOPE. 



magnified, to appear at o o'. The apparent magnitude of the 
image, in such a case, will depend on the angle at which the 
rays from the object enter the eye after traversing the lens ; 
hence lenses of hjgh refractive powers and short focal distance 
magnify most. Such can be used for examining only the most 
minute objects placed very near the eye, since their field of 
vision is extremely limited, and covered by objects of small ex- 
tent. 

309. The magnifying poiver of a lens * may be determined 
very nearly by dividing the limit of distinct vision by the dis- 
tance of the object from the centre of the lens. Thus, in Fig. 
250, if we suppose the former to be five inches, and the dis- 
tance of the object from the centre of the lens, yV of an inch, 
then will the magnifying power be as Jg to 5, which is as 1 to 
50, or as 1 to 2,500 in surface. That is, the length of o o' 
Pi., 251 ^'^ appear fifty times that of 

0'. 

The Magic Lantern, Solar Mi- 
croscope, and Camera Obscura, 
are all difierent forms of the 
simple microscope, and will be 
subsequently described. 

310. The Compound Mi- 
croscope is formed by arranging 
a second lens so as to magnify 
the image of the object formed 
by the simple microscope or sin- 
gle lens. Thus, instead of the 
object itself, the image of that 
object is examined by the eye. 
Fig. 251 shows the principle 

* The diamond affords the most perfect material for magnifying purposes ; 




What kind of lenses magnify most? What class of objects can only be 
examined by such lenses? How may the magnifying power of a lens be 
(loiciaiiufd ? Explain this. How is the Compound Microscope formed ? Explain 
Mils from Fig. 251. 



COMPOUND ^MICROSCOPE. 825 

of the Compound IMicroscope. The object-glass, a b, is fixed in 
the lower extremity of the tube, near the minute object placed 
just below. This presents a magnified and inverted image of 

the object, a" b'\ at the focus 

Fig- 252. i»^u 1 T^ 

ot the eje-glass, c. Bj this 
second lens, or eje-glass, the 
image formed is magnified, 
and brought to the eje, situ- 
ated above and near c, so as 
to appear at a h' of surpris- 
ing dimensions. 

Fig. 252 presents such 
a microscope complete in 
all its parts. The tube, A, 
contains in its upper part the 
eje-glass ; this tube slides in 
a second, B, in the lower ex- 
tremity of which is fixed the 
small object-glass ; B also 
moves in the stand C. Thus, 
by such a movement of these 
tubes, the lenses in them 
may be adjusted to the prop- 
er distance from each other, 
and the object to be exam- 
ined, which is placed at S. 
M is a mirror for reflecting the light from the sun, or a lamp 

for, owing to its high refracting power, it enables the eye to see minute objects 
at a large angle. The diamond, moreover, causes but a comparatively dight 
aberration of the light passing through it, while its dispersive power, or power 
of decomposing light into its prismatic colors, is but trifling, being nearly 
achromatic. For these reasons it is peculiarly adapted to magnifiers °for high 
powers. The sapphire, garnet, and quartz crystal, are also well adapted fur 
magnifiers of high powers. 




Describe the parts of the Compound Microscope when complete, as in Fig 252 
28 



326 WONDERS OF THE MICROSCOPE. 

upon the object at S, and so illuminating and rendering it more 
distinct. This mirror turns on pinions, and can be fixed at any 
desired angle. The various appendages to this compound micro- 
scope are also shown in the figure. D, E, are eye-glasses of difier- 
ent powers ; F, a glass plate, on which minute objects for ex- 
amination may be laid; G, a pair of delicate tongs, for taking up 
small objects : H, a pair of pincers fixed at one end of a wire, 
to the other of which is attached a small ivory capsule for hold- 
ing various objects, the whole turning on a pinion, fitting into 
a hole in S ; J, a delicate point for taking up minute objects ; 
K, the eye-piece for holding the eye-glass; L, a receiver for 
liquids ; 1, 2, 3, small objects fixed on glasses set in sliders, 
which slide between the springs of S. 

311. No instrument of human contrivance has done more 
than the microscope to enlarge the boundaries of knowledge, 
and unfold to the mind the exquisite skill and perfection dis- 
played in creation. It places us in the midst of a world before 
invisible, and which, like a new creation in the freshness of 
beauty, stretches away in enchanting prospects on every side. 
It shows that beyond the limits of our unaided vision all is 
instinct with life, and replete with harmony, skill and wise 
design. 

The most common substances, which afford but little if any 
interest to the unaided vision, under the microscope become 
often objects of the highest interest and instruction. Thus, a 
single grain of marl, for instance, is seen to be composed of 
myriads of flinty skeletons of minute creatures, perfect in their 
organization, and once replete with life and activity, while a 
single drop of liquid may exhibit millions of living animalculoe, 
which appear like huge monsters swimming about and sporting 
at will, as in a vast sea.* By means of the microscope, the 

* According to the microscopic researches of Ehrenberg, a species of slate, 

What is said of the Microscope in reference to human knowledge ? Under 
this instrument, what do some of the most common substances present? Give 
illustrations. 



REFLECTING TELESCOPE. 327 

characteristics and habits of many minute insects, whose oper- 
ations have an important bearing on the convenience and com- 
fort of man, may be learned, and their injurious results thereby 
guarded against. In a word, this instrument enables our vision 
to explore fields of beauty and variety before undiscovered, 
but replete with interest, as bearing upon our physical well- 
being and happiness. 

312. The Telescope^ is an instrument for viewing distant 
bodies on the earth or in the heavens. The kinds employed 
are two, known as the reflecting and the refracting telescopes. 

The Reflecting Telescope. — Fig. 253 exhibits the internal 

Fig. 253. 




arrangement of the more common form of this telescope, known 
as the Gregorian Telescope. In a large open tube is fixed a 
highly polished metallic concave speculum,! ni m', having 
a circular aperture in its centre. The rays, r r\ entering 

found in Bohemia, consists almost entirely of tlie skeletons of minute animals, 
of which forhj-one thousand millions are found to lie entombed within the 
space of a single cubic inch. A species of marl, existing extensively in various 
sections of the United States, is found to be composed almost wholly of the 
bodies of infusoria, well-nigh exceeding in numbers the bounds of credibility. 
So small ai-e these minute creatures, that a thousand might swim side by side 
through the eye of a needle. This marl, thus composed, becomes one of the 
most fertilizing products to be found, and is accordingly extensively used as a 
manure. 

* Ti^?.f, at a distance, oxonlo}, to see. ' 

t A speculum is a reflector formed of highly polished metal. A mirror is 
a reflector made of glass, usually coated on its back with an amalgam of tin 
and quicksilver. 

The uses of the Telescope ? The kinds employed ? Describe the Reflecting 
Telescope, as shown in Fig. 253. 



328 REFRACTING TELESCOPE. 

tlirougli the open end of the tube, are reflected by this specu- 
lum, so as to form an inverted image at i, the focus of the small 
concave mirror, s ; by this small mirror a second erect image is 
formed before the eye-glass, /, at i' ; this image is magnified by 
the eye-glass, i", and is viewed by the eye, at e, the same as in 
the compound microscope. The second lens, at /, is usually in- 
terposed for rendering the rays from the small mirror more con- 
vergent ; this, however, is not necessary ; w w, is a rod and 
screw, connecting with the mirror, s, for regulating the distance 
of this, and adjusting it to the focus of the mirror, m in! . 

Telescopes of the larger size and higher powers have been 
usually reflecting telescopes. =^ These possess some advantages 
over the refracting telescope to be described, such as the avoid- 
ing spherical and chromatic aberration, consequent upon the 
difficulty of obtaining lenses free from these defects. The re- 
flectors of this form of telescope are made of a parabolic form, 
which thus greatly improves their powers of reflecting, and con- 
verging the light falling upon them, and so renders more dis- 
tinct the image of the object. In the reflecting telescope, this 
image is seen erect, and in the same direction with the object. 

Fig. 254 presents an external view of such a telescope 
mounted on a tripod-stand, w^ith a sight attached. 

313. The Refracting Telescope^ for astronomical purposes, 

* The great telescope of Sir William Herschel, constructed under the patronage 
of George III., was a reflecting telescope. The focal length of this was forty feet, 
and had a speculum 49.^ inches in diameter, which weighed 2118 pounds. The 
second reflector at S, Fig. 253, was dispensed Avith, and the waste of light by a 
second reflection thus avoided. The image thus formed was thrown near to 
tlie open mouth of the tube, where it was viewed by an eye-glass directly, the 
observer being seated so as to look into the mouth in front. In this telescope 
many of the larger stars, as Sirius, appeared with the splendor of the sun. 

Lord Rosse's great reflecting telescope was constructed with a speculum six 
feet in diameter, being otherways of proportionate dimensions. 

What are some advantages possessed by reflecting over refracting tele- 
scopes ? How are the reflectors formed ? What is said of the image in this ? 
V.hat does Fig. 254 show? How does the refracting differ from the reflecting 
telescope last described ? 



THE TELESCOPE. 

Fig. 254. 



329 




28=^ 



380 



TERRESTRIAL TELESCOPE. 



differs from the reflecting, just described, in having the image 
before the magnifying-glass formed by lenses instead of mir- 
rors. Fig. 255 shows the internal arrangement of such a 
telescope, the principle of which is similar to the compound 



Fig. 255. 




microscope, differing from that in nothing except the propor- 
tion of its parts. is the object-glass placed at the end of 
the tube, which serves to collect the rays from a distant object, 
and form an inverted image of the same at o o', in the focus of 
the magnifying lens, M; by this it is magnified, as in the 
microscope. Fig. 251, and appears to the eye at E. 

314. The Terrestrial Telescope or Spy -glass. — As the 
Astronomical Telescope, last described, presents objects to the 
eye inverted^ it is unsuited for viewing terrestrial objects; 
accordingly, for this purpose, two additional lenses are inter- 
posed between the eye and the image, whereby the latter is 
made to assume an erect and natural position in reference to 
tlie eye. 

The course of the rays and position of the image in the ter- 

Fig. 256. 




restrial telescope will be readily understood by Fig. 256, where 



Explain Fig. 255. Why is the Astronomical Telescope unsuited for viewing 
terrestrial objects? How is this remedied, and the object made to appear 
upright ? Explain the course of the rays of light, and the manner in which 
the image of the object is seen direct, as shown in Fig. 256. 



TERRESTRIAL TELESCOPE. 831 

is the object-glass and m n the first image formed at the 
focal distance of the converging lens, C ; consequently, the 
rajs from m n, after passing through C, will emerge parallel. 
These then fall on another converging lens, L, of equal focal 
length, by which they are again made to converge and form a 
second image, n m, inverted with respect to the first, but in 
the same position as the object. This image is then vicAved 
through the magnifying lens, M, by the eye at E, as in the 
previous figures. 

These lenses are fixed in tubes, which slide one within the 
other, by which each may be adjusted to the correct focal 
distance. These tubes are better shown by Fig. 257, which 

Fig. 257. 




presents a view of the Mounted Spy-glass. C is for the eye- 
glass or magnifier, in which magnifiers suited to difierent eyes 
may be inserted; b is a shade-glass for protecting the eye 
against the concentrated light from the lens. The great im- 
provements of late, in the construction of large achromatic lenses 
(§ 289), have caused refracting telescopes to be more generally 
employed for astronomical as well as terrestrial observations. 
315. The magnifying power of the telescope depends on the 

What does Fig. 257 show ? On what ratio does the magnifying power of the 
telescope depend ? 



332 GALILEO'S TELESCOPE. 

ratio between the focal distances of the object-glass and the 
eje-glass. Thus, in Fig. 255, suppose the common focus ten 
times nearer the eye-glass than the object-glass, then will the 
instrument magnify ten times ; if fifty times nearer, fifty times, 
and so on. Hence, the magnifying power of the telescope may be 
increased, either by using an object-glass of small curvature, so 
as to throw the image to a great distance, or an eye-glass of 
high curvature and short focal distance. Thus, if the object- 
glass have a focal distance of twenty-five feet (300 inches, for 
instance), and the eye-glass or magnifier one tenth of an inch, 
then will the magnifying power of the telescope be three 
thousand times in diameter, and nine millions in surface. 

Galileo's Telescope. — This is the most simple form of tel- 
escope now used, and possesses some important advantages over 
the refracting telescope, just described. The object in this is 
made to appear erect, by the use of only two lenses. These 
lenses consist of a double-convex lens for the object-glass, and 
a double-concave for the eye-glass ; the latter is placed within 
the focus of the object-glass, so as to magnify its image formed 
before the crossing of the rays. Owing to such an arrangement 
of the lenses, these telescopes are comparatively short, and 
therefore more convenient for many purposes. Opera glasses 
are telescopes of this form. 

According to this how may the magnifying power of the telescope be in- 
creased? Give an illustration. What is said of Galileo's Telescope? How 
does the object in this appear ? How many, and what form of lenses ? What 
are Opera glasses ? 



EXPANSION BY HEAT. 333 



HEAT. 

316. Heat is the sensation experienced when we touch any 
object the temperature of which exceeds that of the human 
body. As in case of light, the principle or essence of heat is 
unknown ; our knowledge of it being confined merely to -the 
effects it produces in matter. 

The sources of heat are various, as that from the sun, from 
combustion, friction, etc. The sun is the chief source from 
whence the earth is supplied with heat, and thus rendered 
capable of sustaining the varieties of animal and vegetable life 
found scattered over its surface. Heat is also freely given out 
whenever oxygen gas, one of the elements of common air, is 
made to combine rapidly with a combustible body, as in the 
burning of the wood and coal of our fires. Friction, caused by 
rubbing together bodies, as pieces of dry wood,* the hands, 
etc., excites heat. This is, moreover, produced by percussion, 
as when metals are hammered upon an anvil, by chemical mix- 
tures, etc. 

317. Expansion of bodies by heat. — Heat exerts an influ- 
ence counter to molecular attraction or cohesion, and, when dif- 
fused through bodies, tends to drive asunder and separate the 
atoms of which they are composed ; hence it is that bodies 
usually expand, and increase in bulk in proportion to the 
amount of heat applied. This increase in bulk of bodies, caused 
by heat, is most clearly shown in the case of liquids and the gases. 

* The American Indians, throughout the whole extent from Patagonia to 
Greenland, formerly procured fire by rubbing together pieces of dry wood until 
they kindled into a flame. Instances have occurred where whole forests have 
been burned down, by fires kindled from the violent friction of the branches 
against each other, caused by the wind. — Parkers Chem. Catechism. 

What is heat ? Do we know anything of the principle of heat ? What are 
some of the sources of heat ? The principal source of heat ? How is heat 
related to molecular attraction or cohesion ? What is said of the expansion of 
different bodies by heat ? 



334 AIR THERMOMETER. 

Experiment. — Pour water or alcohol into a bulb, Fig. 258, 
having a long tubular neck, so as to cause the water to 
rise part way in this, and immerse the bulb in boiling 
w^ater, or apply the heat of a lamp. As the liquid 
within the bulb becomes heated, it will be seen to ex- 
pand and rise in the neck, in proportion to the degree 
of the heat applied. 

Experiment a. — Invert such a bulb, filled only 
with air, and place the extremity of its neck in some 
water. Upon applying the heat of a lamp, the air in the bulb 
will rapidly expand, and be forced out in bubbles through the 
water. This expansion of the confined air will be perceptible, 
with only the heat from the hand applied. 

318. The regular expansion of liquids liid gases by heat 
led to the invention of the Thermometer.,* for measuring 
changes in the temperature of the atmosphere and other bodies. 
The simplest and most sensitive kind of thermometer is that 
discovered by Sanctorio, in 1590, and called, from the manner 
of its construction, the Air Thermometer. 

This is seen in Fig. 259, and consists of a glass bulb, with a 
long stem entering some colored liquid in the small 
vessel beneath. From this bulb, a portion of the air is 
expelled, so as to cause the liquid to rise part way in 
the stem. 

A very trifling change in the temperature of the 
surrounding air will be shown by a corresponding in- 
crease or diminution of the bulk of that confined in the 
bulb, which will cause the colored liquid to rise or fall 
in the stem. To this stem is affixed a scale graduated 
to denote the temperature, corresponding to the height 
of the liquid in the tube. 

* Tliermos^ heat ; metro7i, a measure. 



Give the experiment with water or alcohol ? With air ? The use of the Ther- 
mometer ? What is said of the Air Thermometer ? Describe its construction '! 



MEKCURIAL THERMOMETER. 335 

The extreme susceptibility of air to expansion from heat 
causes such a thermometer, of moderate length of stem, to indi- 
cate the changes of temperature within only a very limited 
range. This was accordingly dispensed with, and thermome- 
ters filled with mercury were substituted. 

319. The Mercurial Thermometer is the form now almost 
universally used. For filling thermometers, no fluid possesses 
the advantages of mercury, since it is not only quite sensitive 
to the effects of a change of temperature, but also expands uni- 
formly, and within comparatively narrow limits. 

The common mercurial thermometer consists of a small glass 
bulb with a stem having a fine bore. This bulb and a portion 
of the stem are filled with pure mercury, by first expelling the 
air, as in § 317, Experiment a, and then placing the end of the 
stem in the fluid. As the bulb cools, mercury flows up into 
the vacant space ; when a sufficient quantity has entered, the 
stem is inverted, and the bulb heated to the 'boiling-point of 
mercury, to expel all the air, and the end of the stem then 
sealed with a blowpipe. 

The bulb, thus filled, is then plunged in ice-water, and the 
point at which the mercury stands in the tube carefully marked ; 
and then again in boiling water, and the point at which the 
mercury stands in this also marked. These constitute the 
freezing and boiling points of the graduated scale affixed. 

The scales of different thermometers are differently graduated. 
That of Fahrenheit (the thermometer commonly used in this 
country) has the freezing-point marked 32°, and the boiling- 
point 212° ; the zero is consequently 32 degrees below the 
freezing-point. The Centigrade thermometer has the freezing- 
point marked 0°, and the boiling-point 100°. Between the 
freezing and boiling points of each thermometer the scale is 

What objections to this form of thermometer ? The kind of thermometer 
now generally used ? Why is mercury well adapted for filling thermometers ? 
Describe the manner of constructing the common Mercurial Thermometer? 
Are the scales of the different thermometers alike ? What are the freezing and 
boiling points of Fahrenheit's scale ? Of the Centigrade scale ? 



336 



PYROMETER. 



divided into the corresponding number of equal divisions, and, 
as the mercury in the bulb expands uniformly, the temperature 
is indicated by the point upon the scale against which it stands 
in the tube. 

320. Solids, like gases and liquids, have their dimensions 
increased by heat. These, however, expand differently with 
the same degree of heat applied. Thus, 800 cubic inches of 
iron heated from the freezing to the boiling point of water, be- 
come 801 cubic inches; 350 of lead become 351, and so of 
other solids. 

The Pyrometer^ Fig. 260, is an instrument for indicat- 



Fig. 260 




ing with considerable accuracy tlie expansion of rods of differ- 
ent metals by the same degree of heat. The rod of metal to 
be tested is placed with one end against a screw, and the other 
resting against a small pin projecting from a right- angular rod, 
so attached to the frame as to turn readily whenever there is a 
pressure against the pin. The free end of this right-angular 
rod is connected with the revolving hand of a graduated disc, 



How does heat aflfect the dimensions of solids ? Illustrate this in the cases 
of iron and lead. What is the Pyrometer ? Describe its construction and 
operation, as shown in Fig. 260. 



TENDENCY OF HEAT TO AN EQUILIBRIUM. 337 

by means of a fine cord passing around its hub, so that, upon 
the slightest pressure against the right-angular rod, the hand 
is made to turn. The other end of this cord is attached to 
a second rod, free to rise and fall, whereby an even balance 
is maintained. If, now, a rod of any metal be placed be- 
tween the screw and pin, and the heat of one or more spirit 
lamps be applied, this will be seen to expand the metal, causing 
the rod gradually to increase in length, as indicated by the 
pressure against the pin, and the corresponding motion of the 
hand. By this means, the different degrees of expansion of 
the various metals may be readily shown. 

In the arts, advantage is often taken of this expansion of 
metals by heat. Thus, the blacksmith places the tire or band 
upon the wheel while the metal is hot, so that, upon cooling, it 
may contract, and draw firmly together the different parts of 
the wood. So, for a like reason, the rivets for steam-boilers 
are fixed in their places while heated, which causes them, when 
cooled, to hold together the overlapping edges with the greatest 
force. 

321. Heat^ like electricity^ tends to an eqiiillhrium, ; con- 
sequently, bodies heated to a temperature above those innnedi- 
ately surrounding, tend to part with their heat to those sur- 
rounding bodies, until a uniform temperature throughout the 
whole is attained. 

Bodies part with their heat in various ways, as by conduction, 
by convexion, and by radiation. If a piece of heated metal be 
laid on other cold bodies, the heat rapidly passes from this, by 
conduction, to these colder bodies, and soon all become of a 
uniform temperature. 

322. Different bodies conduct heat with different degrees 
of readiness ; this power of conduction varies in general with 

What are instances of the advantages often taken of the expansion of metals 
by heat ? What is said of the tendency of heat to an equilibrium ? What are 
some of the ways in which bodies part with their heat? Do all bodies conduct 
heat alike ? How does this usually vary ? 

29 



338 



CONDUCTORS OF HEAT. 



the density of bodies, those more dense conducting it with 
greater facility than those less dense. 

Thus, metals are in general better conductors of heat than 
stones, stones than wood, and wood than air and the gases. 
This is illustrated in many of the implements used about a fire, 
as soldering-irons, pokers, etc., which are usually provided with 
wood handles for protecting the hands. 

It is the superior facility with which metals conduct the heat 
from the hand, that causes a bar of iron, for instance, to feel 
colder to the touch than a piece of wood or cloth, while a ther- 
mometer will indicate the same degree of heat in each. 

rig. 261 serves to show the readiness with which metals 

conduct heat. If a wire 
JPig. 261. gauze be held over tlie 

flame of a lamp, the in- 
candescent - particles, as 
they come in contact with 
the wires, will have their 
heat taken from them so 
rapidly by the metal, as 
to become cooled, and so lose their glow upon passing through 
the wires. Thus the flame will be intercepted, as at A, instead 
of passing the screen, as shown at B. 

Of imperfect conductors of heat, common air, a type of the 
gases, afibrds one of the best examples. It is from this cause 
that clothing formed of loose fibrous materials, as wool, furs, 
etc., which hold between their particles large quantities ctf air, 
is warmer than that formed from the more compact fabrics of 
cotton and linen. It is from a like cause that snow serves to 
prevent the escape of heat from the ground, and so protects 
vegetation against the severe frosts of winter ; this holding be- 




Why are many of the implements used about a fire provided "with wood 
handles ? Why does a bar of iron feel colder or warmer to the hand than a 
piece of wood or cloth? What is said of air as a conductor of heat? Why 
are furs and woolleus warmer than cotton and linen fabrics ? 



CONDUCTION OF LIQUIDS. 339 

tween its particles a large amount of air, and so forming a very 
bad conductor of the heat. 

323. Liquids are among the ivorst conductors of heat. — 
Indeed, so slight is their conducting power, that Count Rum- 
ford, after a great variety of experiments, was led to infer their 
perfect non-conducting power of heat. Later experiments 
have, however, shown them to be conductors of this to a very 
limited extent. Fig. 262 exhibits an arrangement for showing 

this. A tunnel-shaped vessel, nearly filled with 
water, contains a delicate air-thermometer, with 
its stem passing down through a cork into the 
small bottle beneath ; this thermometer will read- 
ily detect the least change in the temperature of 
the water. Upon the upper surface is poured 
some ether, which, when inflamed, as seen in the 
figure, produces a great amount of heat. Such 
a heat, thus applied, is found not to afiect in any 
perceptible degree the temperature of the liquid 
immediately below its surface, as may be shown 
by raising the bulb of the thermometer very 
near the surface of the water, next the burning 
ether, when it will still remain unaffected. 
Liquids become heated by convexion.* — Thus, in a vessel 
of water, heated over the fire, those particles of the liquid rest- 
ing against the heated metal of the bottom become hot and rise 
while others above sink down and occupy their place, and in 
turn become heated and rise, and so the process goes on until - 
all the particles of the liquid are brought in contact with the 
heated surface. In a similar manner air surrounding a heated 
body becomes warmed. 

324. Bodies radiate their heat in all directions. — Radi- 

* Corii to, and veho^ to bear. 

What is said of liquids as conductors of lieat ? What is Fig. 262 designed 
to illustrate ? Explain this. How do liquids become heated by con-vexion ? 




340 



REFLECTION OF HEAT. 



ant heat follows in most respects the same laws as light from 
luminous bodies, being emitted in direct lines from all sides of 
the heated body, and subject to reflection, refraction, and polar- 
ization, the same as light. Different surfaces radiate heat with 
different degrees of facility. Dark and rough surfaces are bet- 
ter radiators of heat than smooth and polished surfaces ; for 
this reason, stoves of cast-iron are better warmers than 
those of polished sheet-iron. So, for a like reason, vessels, as 
tea and coffee pots, for retaining the heat of the liquids they 
contain, will do this much better if made of smooth and pol- 
ished metals. 

The rays of heat incident on a concave reflector may he 
converged to a focus the same as light. — This maybe shown 
by an arrangement seen in Fig. 263, where a and b are two 

Fig. 263. 



2*<'-'-l> 




^ 



brightly polished copper reflectors, of the parabolic form, placed 
exactly opposite each other, and in the same line, at a distance 
of from ten to twenty feet. Let these stand in a dark room, 
and in the focus of a place a ball of cast-iron, heated a little 
below redness. The rays of heat radiated from this upon a 
will be reflected and fall on 6, by which they will be converged 



What laws does radiant heat follow ? Do all surfaces radiate heat with the 
Bame facility ? What surfaces do this with the greatest facility ? What is 
said of liquids heated in brightly polished vessels ? What is said of the re- 
flection of heat ? Explain this by Fig. 263. How may certain combustibles 
be ignited without any visible cause ? 



COLD, HOW CAUSED. 341 

to a focus. The bulb of a thermometer placed in this foeas 
will show a powerful heat ; and tinder, phosphorus, and gun- 
powder, placed in the same position, will be inflamed without 
any visible cause, since no light appears. 

325. Ti^ansmission of Heat. — Heat and light, as we have 
already remarked, are independent agents, although governed 
in most respects by similar laws. In the facility with which 
they traverse various substances, the rays of heat and light 
differ widely. Many substances which are transparent to light 
intercept the passage of heat, while many which intercept the 
light transmit heat freely. Thus, if a piece of plate-glass, 
which allows of the ready transmission of light, be placed 
before a fire, it will intercept the rays of heat, and become 
soon heated, while a crystal of rock-salt, of equal thickness, 
placed at the same distance from the fire, becomes scarcely 
warmed. Such bodies as plate-glass, which allow the passage 
of light, are said to be diaphanous, while those, as rock-salt 
crystals, which transmit heat freely through them, are termed 
diathermous. 

326. Cold and its Causes. — Cold is simply a negative 
term, and implies the absence of heat. This is produced when- 
ever bodies change from a denser to a rarer state. In such a 
case the capacity of the body for holding heat is said to be in- 
creased, so that the quantity of this sufiicient to be rendered 
sensible in the denser state is absorbed and disappears or be- 
comes latent when the body assumes a rarer state. This heat, 
which is absorbed, or becomes latent, is again given out and 
rendered sensible, when the body is once more compressed and 
made to occupy its former limits. 

This capacity of bodies for heat may be illustrated by a 

In wliat do the rays of heat and light dififer widely ? Illustrate this by 
pieces of plate-glass and rock-salt crystal placed before a fire. What term is 
given to substances Tvhich allow the light to pass through them readily ? 
What to those that aUow heat to pass through them ? What is cold ? When 
produced ? Why is cold produced when bodies pass from a denser to a rarer 
state ? 

29* 



342 COLD PRODUCED BY EVAPORATION. 

sponge containing water. If the sponge be compressed, the 
water will flow out and become perceptible, but upon removing 
the pressure and allowing it to expand, its capacity for holding 
water will be increased so as to absorb this and appear compar- 
atively dry ; upon again compressing it, the latent water con- 
tained in it will be forced out and again rendered percept- 
ible. 

327. Cold produced by Evaporation. — The capacity of 
liquids for holding heat is greatly increased when they are 
made to assume a state of vapor, and hence it is that evapo- 
ration is a cooling process. Thus, a bowl of any hot liquid 
placed where the evaporation may go on freely, soon becomes 
cooled, owing to the escaping vapor carrying away from the 
lie-uid its heat. 

Thus it is that sprinkling the floor in a warm and dry day 
renders it cool, since the increased capacity of the vapor for 
heat takes it from the floor. From the same cause damp 
clothes, owing to the vapor escaping from them, rapidly take 
the heat from the body, and reduce its temperature. Hence 
the danger from taking cold by wearing such. 

This may be illustrated by the Pulse-glass, Pig. 264, which 
consists of two bulbs blown upon the 
Fig. 264. extremities of a glass tube, and con- 



^ 



^^r; Sy^ taining some colored water or alcohol ; 

from the space above the liquid the 
air has been expelled by heat, and the glass then sealed so as 
to leave a vacuum within. If one of the bulbs be grasped by 
the hand, the warmth of this will cause evaporation to proceed 
rapidly from the liquid in the other bulb, which will absorb the 
heat from that in the hand, and produce a sensation of cold 
quite perceptible. 

The cold produced by evaporation may be illustrated by an 

How may the capacity of bodies for holding heat be illustrated ? How does 
evaporation produce cold ? Why does sprinkling the floor in a warm day cool 
it .' What is the Pulse-glass? How is the cold produced by this occasioned ? 



WATER FROZEN BY ITS OWN EVAPORATION. 343 

arrangement seen in Fig. 265. A glass tube, with a bulb upon 

each end, and bent as seen in the figure, is in- 

'^' ^^ ■ serted in an air-tight brass cup, covering the top 

of a receiver, resting on the plate of an air-pump. 

These bulbs contain a small quantity of pure 

water, which, when placed for an experiment, 

should be allowed to flow into that upon the bent 

extremity of the tube. The other bulb within 

the receiver has a covering of cotton placed 

around it. 

* v /» Experiment. — Dip the cotton in some sul- 

\ 4b)^ I phuric ether, and with a small cup placed over 

the hole of the air-pump, to prevent the liquid 

ether entering this, place the cup, with its glass, tightly upon 

the receiver, well fitted to the pump-plate. Exhaust, and the 

vacuum formed will hasten the evaporation of the ether * from 

the cotton, and the cold thus formed will condense the vapor of 

the water within the lower bulb ; this will produce a vacuum 

over the water in the upper bulb, causing this to evaporate with 

such rapidity as to freeze in four or five minutes. A person 

may be well-nigh frozen in a warm day by winding about the 

body a sheet dipped in ether. 

The same results may be produced by plunging the lower 
bulb in a freezing mixture of salt and snow. This instrument 
was invented by Dr. Wollaston, and named by him the Cry- 
ophorus^ or frost-bearer. 

328. Water Frozen by its own Evaporation. — A most re- 
markable instance of intense cold, produced by evaporation, is 
shown by the following celebrated experiment first performed by 
Leslie. 

* With an air-pump, designed for nice experiments, this with ether should 
never be performed, since the vapor of the ether acts on the yalves, and 
seriously injures them. 

Explain Fig. 265. Give the experiment with this apparatus. 



344 



WATER FROZEN BY ITS OAVN EVAPORATION. 



Experiment. — Place upon the plate of the air-pump, Fig. 
266, a broad and shallow basin of sulphuric acid. Over, and 



Fig. 266. 




just above this, upon a wire-stand, place a shallow metallic 
cup filled with pure water. Cover all with 
a glass receiver well fitted to the pump- 
plate, as seen in the figure. Exhaust, 
and produce a vacuum in the receiver; 
vapor will now escape rapidly from the sur- 
face of the water, and be absorbed by the 
sulphuric acid ; this w^ill take the heat from 
the water with such rapidity as to freeze it in 
from thirty to sixty seconds. 

Fig. 267 is a convenient apparatus for 
showing, by means of a thermometer, the 
sudden change in the' temperature of the 
water during the progress of evaporation 
in a vacuum, as just described. The bulb 
of a thermometer, suspended by a sliding-rod 
in the long neck of the receiver, may be 

Give the experiment for showing the manner in which water may be frozen 
by its own evaporation in a vacuum. For what is the apparatus seen in Fig. 
267 designed? 




FREEZING MIXTURES. 345 

lowered into the water in the cup over the basin of acid, as 
seen in the figure.* 

Freezing Mixtures. — Whenever a solid is rapidly con- 
verted into a liquid, intense cold is produced. This may be 
shown by mixing together salt and snow, or pulverized ice, 
which mutually dissolve each other, and take the heat from the 
surrounding objects. In this manner creams are frozen during 
the warm days of summer. 

By mixing together three parts of chloride of lime and two of 
snow, a cold of 50° below 0° may be produced, and mercury 
thereby frozen. The greatest artificial cold yet produced (175° 
below 0°) is from the rapid evaporation of solidified carbonic 
acid gas in a vacuum. 

* This experiment is one of the most rigid tests of the air-pump now adopted. 
If a pump -will freeze a small cup of water in from one-half to three minutes, 
without the aid of ether or other evaporating liquids, its operation may be 
deemed satisfactory for all the varieties of pneumatic experiments. An air- 
pump in possession of the author has repeatedly performed this experiment in 
less than one minute. 

How may freezing mixtures be prepared ? Cause of the cold produced by 
these ? What is said of the cold produced by a mixture of chloride of lime 
and snow ? By what means has the most intense cold yet known been pro- 
duced ? 



346 MAGIC LANTERN. 



ADDITIONAL PHILOSOPHICAL INSTRUMENTS. 

329. The Magic Lantern. — This is an interesting form 
of the single microscope adapted for exhibiting, on an extended 
scale, pictures painted on glass, and natural objects more or 
less transparent to light. The object, in this case, is highly 
illuminated by an artificial light, and its image, greatly magni- 
fied, is thrown on a screen more or less distant from the lens. 
Thus this image is viewed by the eye, instead of the object 
itself, as in the common form of the simple microscope. 

The Magic Lantern, in the early periods of its use, like most 
other philosophical machines, was employed as a mere toy for 
amusing children. The earlier forms of this instrument were 
exceedingly rude and imperfect, consisting simply of a hemi- 
spherical condensing lens fixed in a side tube, and a magnifier 
placed in a second tube sliding in the first, which extended 
from the side of a tin canister, while before both of these lenses 
was placed the picture to be magnified. With such an apparatus 
but little dignity or interest attached to these early " shows." 
As improvements were, however, subsequently made in the 
construction of this instrument, it came to be employed for 
exhibitions of greater utility and interest, until it may now be 
regarded as well-nigh mdispensable to popular lecturing.* 

The convenient and attractive form of exhibitions with the 
Magic Lantern has caused this instrument to be extensively em- 
ployed by teachers and lecturers on popular science. We shall, 
therefore, devote a brief space to directions in regard to its 
uses and liabilities, under the various forms in which it is 
employed for popular exhibitions. 

* No lecturer has done more than Dr. Lardner to dignify the Magic Lantern, 
and show its real utility for scientific and other illustrations. The striking 
views with this instrument will be remembered by those who listened to the 
instructive lectures delivered by this distinguished philosopher, a few years 
since, in this and other cities of our country ; contrasting as they did with the 
too many mere tawdry shows with this instrument, given by those who have 
neither science nor the requisite mechanical skill. 



MAGIC LANTEEN. 



347 



Fig. 268 presents a sectional view of the internal structure 
of the Magic Lantern. A is an argand lamp, which is placed 



Fig. 268. 




in a tin box or case, and may be fixed in its proper position ; 
this lamp is supplied with air through a circular opening in the 
bottom of the lantern, at 0. C is a conical glass chimney, for 
giving steadiness to the flame, and for conducting the heated 
gas out through the opening, W, which is bent so as to pre- 
vent the escape through it of the inclosed light. 

R R is a metallic reflector * for tln'owing the light radi- 

* In this position of the reflector, the lamp standing within its focal 
point has its light reflected diverging upon the condensers ; by this ar- 



348 MAGIC LANTERN. 

ating towards it back upon the large convex lenses, L L. 
These lenses, known as condensers^ are fixed in a short tube 
within the lantern, and serve to condense the light falling 
on them on the object painted upon the sliders ; these sliders 
move through a narrow slit, N N, at the head of the larger 
external tube, and are held in their proper positions by 
springs. 

The light from the illuminated object falls on the mag- 
nifying lenses, M M (§ 286); by these an enlarged image 
is formed on a screen before the lantern. D D is a movable 
diaphragm^ with a circular aperture, for the purpose of cut- 
ting off such scattering rays as tend to injure the distinctness 
of the picture. The magnifying lenses are fixed in a smaller 
tube sliding in the larger, which may be moved nearer or 
farther from the object to suit the focal distance. The whole 
internal surface of the lantern and tubes is painted black, to 
prevent any reflection of the irregular light. 

330. Directions for the use of the Magic Lantern. — The 
lamp should be w^ell trimmed before using, and filled with the 
best sperm or lard oil; the wick being even and raised as high 
as may be without smoking. 

The reflector, glass chimney, and lenses, should all be re- 
moved before an exhibition, and carefully wiped; with the 
lenses a piece of clean buff-leather or silk handkerchief may be 
used, and a circular movement given in wiping. Avoid getting 
finger-marks on these, and see that they are replaced in their 
proper positions, as shown by the cut. 

The lantern should be placed on an elevated stand (See Fig. 
271), at distances from the screen varying from eight to twenty- 
five feet, according to the focus of the magnifier, the degree of 

rangement a greater quantity of light is made to fall on the condensing lenses 
than by the former method, where the reflector was fixed upon the rear sur- 
face of the lantern, so as to converge the less amount of light falling upon it on 
the lenses. These reflectors should be made of a parabolic form (§ 270). 
Much depends on their proper form and position. 



MAGIC LANTERN. 849 

the illumination, and the size of the circle which it is desired 
to form on the screen. At the splendid exhibitions with the 
Oxhydrogen Microscope, given by Mr. Whipple at the Tremont 
Temple, in this city, in 1850, the lantern was placed at a dis- 
tance of sixty feet from the screen. The author of this work 
has recently assisted in arranging an extensive apparatus for 
exhibiting the dissolving views, where the lanterns were placed 
about forty feet from the screen. About ten feet is the usual 
distance. As the same amount of light is thrown on a small 
as on a large circle, the brilliancy of the picture formed on the 
screen will, of course, diminish as its magnitude is increased. 
With the illumination from an oil lamp, a circle of eight or ten 
feet diameter is sufficiently large. With the Drummond Light, 
one of fifteen or even twenty feet may be formed. 

The position of the lamp in the lantern should be such as, 
when lighted, to cast upon the screen a rvell defined and 
uniform circle of light. ^ In some lanterns this position of 
the lamp is fixed ; in others it is determined by an adjusting- 
screw upon the outside, which moves it to and from the con- 
densing lens, as may be required. 

The sliders are placed in the slit with their pictures inverted. 
These should be free from dust, and arranged in a box, so that 
they may be readily taken up in the order in which it is desired 
to use them. The magnifying lens may be adjusted to its focal 
distance from the painting by means of the sliding-tube in 
which it is fixed. During the exhibition no light should ])e 
allowed to escape into the room, except that passing through 
the lenses upon the screen. 

The position of the lamp and lenses should be carefully 
adjusted before the lecture, and the lantern allowed to stand 
for some time previous in a warm room, so that the lenses may 
become w^armed, and thus prevent the moisture condensing on 
them, and so injuring their transparency, as is often the case 
in cold weather. 

* See that the lenses of the inner and outer tubes are in a direct line. 

30 



350 PHANTASMAGORIA. 

331. The Screen on which the picture is thrown should be white. 
A plastered wall, hard finished, or piece of cloth twelve or 
fifteen feet square, well covered with white paint, and attached 
to a roll, forms a suitable screen. Where the screen is inter- 
posed between the spectators and the operator, as in producing 
the phantasmagoria, it should be rendered as transparent as 
possible. Such may be made by smearing over bleached cotton, 
or linen, with a coat of white wax, or by using muslin or 
bleached linen stretched on a vertical frame. Wetting occasion- 
ally with water during the exhibition, improves the transparency 
of the two last ; this may be done by a small syringe. 

Since nearly half the light is intercepted and prevented from 
passing through such screens, the picture formed on them is, of 
course, much less brilliant than those formed on opaque screens 
prepared as already described. These transparent screens, how- 
ever, possess the advantage of allowing the operator and lantern 
to be concealed from view, causing the picture to appear sus- 
pended in the air, and so rendering the illusion more perfect. 

332. The Phantasmagoria. — This singular illusion is pro- 
duced by the formation of an image on a transparent screen, as 
we have just intimated. In this case the operator is concealed 
from view, and as the screen is not seen, the images formed 
on it appear suspended in the air. 

Let the operator, standing quite near the screen, and holding 
the lantern under his left arm, with his right hand adjust the 
focal distance of the lens so as to form a distinct picture on 
the screen ; then moving slowly from this, and at the same 
time regulating the focus of the lens, the picture will gradually 
increase in magnitude, and appear as if approaching the specta- 
tors ; upon returning again towards the screen, the picture will 
appear to recede. If the lenses and quantity of light be prop- 
erly regulated the illusion will be complete, and the efiect 
most wonderful. 

333. Dlssolvmg Views. — These are most extraordinary magi- 
cal efiects, produced by placing two lanterns of equal power so as 



DISSOLVING VIEWS. 



851 



to throw two pictures of equal magnitude, and in the same 
position, on the same space of the screen. The lanterns are 
placed side by side on a stand, as shown in Fig. 269, and may 

be adjusted to the proper 

^^* * angle with each other 

^^^^^^^^^k by turning on pivots in 

B^^B^pl^ front, and, when so ad- 

HPlI IPI justed, made permanent 

■ i' ^ by binding-screws. A 

diamond-shaped shade, 

^"^^^"^ S, slides in a groove in 

=^^=_-^. front of the lanterns, 

^ and is so proportioned 

^i .--^' that its widest part, 

"'^j ^ when directly before one 

of the nozzles, shall 

wholly intercept the light 

^^ ' J^^ through that, while its 

^^ _si^ - -^ ^-^-^ ~"" point just reaches to the 

outer edge of the other 
nozzle. Thus, in moving this sliding-shade. the light from one 
lantern is cut off from the screen just in proportion as that from 
the other is let on.* 



* Dissolving Stop- Cock. - 



Fig. 270. 



In the recent arrangement of these dissolving 
lanterns, where the Drummond 
Light (see § 334) is used for 
illuminating, instead of the slide 
S, a Dissolving Stop-Cock, Fig. 
270, is employed, for alternately 
letting on and closing off the 
Ught. 

The cylinder of this is fixed 
upon the rear part of the stand, 
between the lanterns. The tube 
from the oxygen bag is screwed 
to the stop-cock, 0, that from 
the hydrogen bag or generator, to H. From the stop-cocks, o o', lead small rub- 
ber tubes for conveying the oxygen to the compound blowpipes arranged 




352 DISSOLVING VIEWS. 

With the lanterns and lights properly adjusted, let two sliders 
be placed in the slits, one for instance representing a landscape 
by day, and the other precisely the same landscape by night ; 
and let the light through the nozzle which contains the land- 
scape by day be unobstructed, while that through the other 
is intercepted ; the picture on the screen will then represent 
the landscape by day. If the shade be now slowly moved, the 
nozzle of the lantern which shows the day-landscape will begin 
gradually to close, while that which shows the night-landscape 
will gradually open. The effect will be that the daylight will 
gradually decline upon the picture, and the objects represented 
will assume by slow degrees the appearance of approaching 
night. The gradual change will go on until the nozzle of the 

in either lantern, as seen in Fig. 271 ; from h h lead the hydrogen tubes for 
conveying hydrogen to these same blowpipes. K is a key, nicely fitted, to 
turn in the cylinder ; through this key, opposite and H, are two holes, which 
connect, one with the stop-cocks o o', the other with h h', and in such a man- 
ner that when the lever of the key is turned towards the side h o, the blow- 
].ipe of the lantern connecting with these will be fully supplied with the 
requisite proportion of gases from and H, while these gases will be cut off 
from h' o' ; and so, when the lever is turned in the opposite direction, the gases 
flow through h' o', but are cut off from h o. 

Thus the Drummond Light is gradually produced in one lantern, while at 
the same time it is as gradually closed off in the other, and so causing a most 
wonderful dissolving effect. To prevent the jet of gas from being wholly ex- 
tinguished, so as to require re-lighting at each turn of the key, two fine slits 
are made on this upon each side of the hydrogen hole, whereby this gas, which 
is the combustible gas, may be allowed to flow, in a minute quantity, through 
the jet from which the oxygen is cut off, and so support the flame. To regu- 
late the distance to which the key may be turned without extinguishing the 
hydrogen jet, a small pin is fixed in the side of this key, which strikes against 
two others set at the proper points on the upper end of the cylinder. 

The stop-cocks, H, may be left entirely open, and the requisite amount 
of gases for each be regulated by the stop-cocks upon the opposite side of 
the cylinder. 

For illuminations with the Drummond Light, no arrangement now in use 
equals this in point of economy and convenience. For the preparation of the 
gases and regulation of this light, see a future section. Near the stop-cocks, 
H, the cylinder is stamped with these initials, for indicating to which the 
oxygen and hydrogen tubes should be attached. 



njii 



'^:^ 
.^ 




OXHYDROaEN MICROSCOPE. 353 

lantern containing the day-picture is completely closed, and 
that contaming the night-picture completely open, when the 
change from day to night will have been completed.* So 
other scenes may be made to dissolve imperceptibly one into 
the other, f 

334. The Oxhydrogen Microscope. — This, in principle, is 
similar to the Magic Lantern, and is used in connection with 
that instrument ; the object being intensely illuminated by the 
rays from the Drummond Light, J concentrated upon it by the 
lenses of the lantern, and then magnified by a powerful magni- 
fier, so as to form on a screen an image of huge proportional 
dimensions. 

Fig. 271 shows a proper arrangement of this apparatus. A 
small ^cylinder, i, of quicklime, is placed in a movable socket, 
and so adjusted that an ignited jet of oxygen and hydrogen 
gases shall blow through the compound blowpipe, ^, against 
its upper extremity, on the side next the lenses. This produces 
an intense light, which, falling on the condensing lenses, c c, 
and then on those at c c, is converged by these last on the mi- 
nute object at o, situated just without the focus of the magnify- 
ing lens, 7n. This object, thus powerfully illuminated, is then 
magnified by ???, and its image thrown upon a white screen, as 
shown at S S. 

The gases supplying the jet pass from the oxygen bag and 
hydrogen generator, placed beneath the stand, up through the 
small rubber tubes, meeting and commingling just as they 

* Lardner. 

t The pictures should be of the same size, and placed exactly in the centre 
of the lenses. 

t The Drummond Light, so called from its discoverer, is formed by the in- 
candescence of a small cylinder of quicklime placed in the jet of the oxhydro- 
gen blowpipe. This is one of the most brilliant artificial lights known. The 
lime cylinders used with this light, except when in use, should be kept free 
from air in a small bottle provided with a tallowed cork ; in this way, when 
purified hydrogen is used, the same cylinder may be made to afford a light 
through two or three exhibitions. During the exhibition the cylinder should 
be occasionally turned in the tube, so as to present a fresh surface to the jet, 

30* 



354 OXHYDROGEN MICROSCOPE. 

escape from the extremity of the compound blowpipe, p. where 
they are ignited.* 

These rubber tubes enter the lantern through a circular 
opening at the bottom, and a corresponding opening in the wood 
stand on which is fixed the blowpipe arrangement. The tubes 
conducting the gases from the oxygen bag and hydrogen gen- 
erator connect with the body of the blowpipe by means of two 
gallows-connectors (Fig. 58) ; that conveying the oxygen 
should connect with the inner or central tube of the compound 
pipe, p. To prevent the possibility of a mistake, the screw- 
holes for the gallows-connectors have marked beside them, H, 0, 
corresponding with the hydrogen and oxygen tubes. The quan- 
tity of the flow of these gases may be regulated by the two 
small stop-cocks in the tubes beneath the lantern. f 

The luminous point of the lime should be directly in a line 
with the centre of the lens, and near its focus. If this 
point deviate only a trifle from the exact point required, the 
circle of light on the screen will be defective ; this position 
may be obtained by means of regulating-screws in the wood- 
stand. 

The directions in regard to the lenses, the distance from the 
screen, and the luminous circle described on this, are the same 
as for the magic lantern. The objects, such as portions of flies' 
legs and wings, cheese and fig mites, bees' stings, portions of 

* This blowpipe is formed from two small copper tubes, one 
Fig. 272. within the other, connecting at their lower extremities with the 
two rubber hose. Thus, the gases (which, when mixed, form an 
CKplosive compound) do not mingle until just as they escape from 
the ends of the concentric blowpipe. By this form of the jet 
all possibility of an explosion is removed. Fig. 272 shows an 
arrangement of the concentric tubes of this compound blow- 
pipe, where the openings are shown of the usual size. 

t In the arrangement of the oxhydrogen microscopes now made 
by Mr. Chamberlain, of this city, the gallows-connectors are dis- 
•'*''» r pensed with, and in their place are two small stop-cocks connect- 
•* » ing the rubber tubes directly with the body of the jet. This dis- 
penses with the small stop-cocks beneath the lantern, and so facili- 
talcs the regulation of the flow of the gases of the jet. 






PREPARATION OF OXYGEN GAS. 355 

hairSj animalculae in water,* etc., are fixed in circular apertures, 
on strips of thin plate-glass, or confined in cavities between 
this, as shown in Fig. 252. 

The oxhjdrogen microscope is peculiarly well adapted for 
popular evening exhibitions, since the images of minute living 
objects may be formed on a screen, before the audience, of sur- 
prising magnitudes. Thus, while the common form of micro- 
scope allows of the objects being viewed by only a single eye 
at a time, the oxhydrogen presents the same at once to a whole 
assembly. 

The size of the image varies with the distance of the magni- 
fier from the screen, and this distance may be increased in pro- 
portion as the illumination is more intense. Thus, minute 
objects may be magnified in surface many millions of times, 
so that the image of a flea or a louse shall appear in magnitude 
equal to an ox or elephant, and the animalculae of a drop of 
water like huge monsters swimming in a deep ocean. 

The 'preparation and use of the gases for forming the 
Drummond Light. — This light, as already intimated, is 
formed by the incandescence of lime produced by an ignited jet 
of oxygen and hydrogen gases, mingled in certain definite pro- 
portions ; these proportions being about two parts, by volume, 
of hydrogen to one of oxygen, as in the formation of water, of 
which they are the elements. 

Oxygen gas, besides being the chief element of water, is 
also the vital principle in common air, and enters very largely 
into the composition of various earths and salts. This gas sup- 
ports combustion, causing bodies to burn in it with surpris- 
ing energy, but does not itself burn. It may be prepared from 
a great variety of substances, and in a variety of ways, through 
the agency of heat or acids. We shall, however, describe only 
two methods, which, in our judgment, combine in the highest 
degree convenience and economy. 

* Ordinary well-water, or water that lias been subjected to considerable 
prtssure, contains few if any animalculse. These are best sbown by water 
from a pool exposed to sunlight. 



356 



OXYGEN, HOW PREPARED. 



The first and preferable method of making oxygen is from 
cl dor ate of potash^ a salt largely composed of this gas, and 
which yields it in great abundance and purity when heated. 

Fig. 273 exhibits a convenient apparatus for preparing oxygen 

Fig. 273. 




for the Drummond Light, or other purposes. C is a copper 
flask, varying in capacity from one pint to a quart, provided 
with a plug, 5, which screws into its nozzle against an air-tight 
shoulder ; to 5 is attached, by soldering, a brass tube ; over 
the end of this tube, at ^, a proper distance from the heated flask, 
is slipped an elastic rubber hose ; the other extremity of this 
hose is also slipped upon the end of a glass tube, 7^, bent at a 
right angle, and extending down about an inch below the sur- 
face of the water, w, in the Woulf's bottle or purifier, r is a 
safety-tube of glass, also passing down through a tightly-fitting 
cork into the water. From the outer end of a second bent glass 
tube, 6, leads another hose, connecting with the stop-cock 
of the large rubber bag, 0. Beneath the copper flask is a sheet- 
iron chimney, A, which serves to conduct the flame from a spirit- 
lamp, on which it stands against the bottom. 



OXYGEN, HOW PKEPARED. 



367 



Directions for making Oxygen. — Expel the air as far 
as possible from the rubber bag by rolling it up compactly, and 
then close the stop-cock attached to its nozzle ; remove the 
screw-plug, s, and pour into the copper flask from five to eight 
ounces of chlorate of potash,* well mixed with two or three of 
powdered oxide of manganese ; see that the screw and face of 
the neck are free from any dust, and then replace the plug. 
Connect the hose leading from t with the purifier, and also that 
from the rubber bag with the same, at b ; place a spirit- 
lamp f beneath the flask, as shown in the figure, and, with the 
stop-cock open, let there be a free communication through 
the water of the purifier into the bag. 

In five or ten minutes the potash will melt, and form with 
the manganese a semi-fluid compound, when a decomposition of 
Fig. 274. the former will soon commence, and the 

gas pass over with a free and uniform flow, 
being cooled and purified in its passage 
through the water. When the bag is filled, 
or the gas ceases to puss over, remove the 
lamp, slip the hose from t^X ^^^ close the 
stop-cock. The gas is now ready for use, 
and, when wanted for the Drummond Light, 
may be attached, as in Fig. 271. If ju- 
diciously used, twelve or fifteen gallons of 
oxygen will supply a suagle light from one 
to one and a half hours. 

A second method of preparing Oxy- 
gen Gas, is by heating powdered per- 

* Every ounce of this salt -will yield about one and a half gallons of oxygen. 
This, however, depends on the purity of the potash, which should be kept in 
close jars, and free from the moisture of the atmosphere. Good chlorate of 
potash usually has a thin scaly appearance, and a shiny lustre. 

t These alcohol lamps are provided with a ground-glass cap, fitting tightly 
over the wick. When the lamp is not in use this cap should be kept on. 

t Guard against removing the lamp and allowing the flask to cool while con- 
nected with the bottle ; for, in such case, the vacuum formed in the flask will 
cause the water to flow over into this. After using, pour into this copper 




358 



OXYGEN. HOW PREPARED. 



Fig. 275. 



oxide of manganese in a cast-iron bottle, Fig. 274. This bottle 
is of the same capacity of the copper flask, 
and has a tube, ground to fit tightly its 
neck ; from this tube leads a hose con- 
necting with the purifier. Fill the bottle 
about half full of manganese, and place in a 
coal fire. At a red heat the manganese 
will part freely with its oxygen, which 

J 11 will flow over as from the chlorate potash 

/^Ibs. •i^^* described, but in less purity than from 

^^Jf that salt. 

335. Fig. 275 shows a small portable 
stove, convenient for holding the bottle 
in the manufacture of oxygen. With this 
bottle oxygen may be also made from chlo- 
rate of potash, requiring, however, the heat 
of a coal fire. 

The hydrogen for the Drummond 
Light may be used directly from the gen- 
erator, or after passing through the puri- 
fier,* as seen in Fig. 271. Occasionally, however, it is conven- 
ient to prepare the gas previously, and use from a second bag 
like that for the oxygen in that figure, f 

flask, while hot, two or three gills of water ; this will dissolve the manganese, 
etc., which has become hard, and allow it to be poured out ; in this way the 
flask may be easily cleansed ; if allowed to stand a while and cool, the process 
of cleansing becomes difficult. Guard against piercing the soft copper by 
sharp sticks, wires, etc. 

* This, as well as oxygen, carries over with it certain volatile acids and other 
impurities, which are stopped and received by the water. 

t The gas, when burned from the compound blowpipe, should be forced 
from the bags by a moderate pressure. For obtaining this pressure a wide 
board may be used, with one end resting on the floor, and sustaining a weight 
of forty or fifty pounds. This bag may be placed in a box beneath the stand, 
and a pressure better applied, if desired. Guard against getting this in any way 
pierced. Experience will alone teach the necessity of proper care in all exper- 
iments with these gases ; mere written or verbal cautions seldom impress the 
careless operator sufficiently to insure proper care. 




HYDROGEN, HOW PREPARED. 



359 



The Hijdrogen Generator, Fig. 276, is a convenient ap- 
paratus for the rapid manufacture of this gas. This consists 
of a cylindrical copper cistern, holding from two to seven gal- 
lons, provided with a wood cover, held firmly to the cistern by the 
binding screws, 5 s. and having a stop-cock, c, extending down 
through its centre, to the lower extremity of which, at «, screws 
a copper bell.* Within this bell is 
suspended a copper bucket filled 
with granulated zinc, or, which is 
preferable, a roll of sheet zinc, Z, 
resting on cross- wires, hooked to 
a main wire fastened to the cap 
of the bell. A rubber hose, t, is 
attached to the upper end of the 
stop-cock c, and leads ofi" to the 
purifier or other vessel, as seen 
in Fig. 27T. 

336. To prej)are the Hydro- 
gen Generator. — Fill the cistern 
nearly half full of water, into 
which pour about one-fifteenth its 
bulk of sulphuric acid, and mix 
well ; lower the bell with its zinc 
into the liquid, and secure the cover by the binding-screws ; 
open the stop-cock, and allow the liquid to rise in the bell, and 
then close it again. The action of the acidulated water on the 
zinc will evolve hydrogen rapidly, and expel the liquid from the 
bell ; f as soon as this is forced below the zinc, as shown in 
Fig. 276, the action will cease. The stop-cock may now be 




* A leather-washer should be placed on the cap of this bell, between this 
and the wood, to avoid the possibility of a leakage. 

t The rationale of this is as follows : The water is decomposed ; its oxygen 
uniting with the zinc forms an oxide of zinc, while the hydrogen, the other 
element of the water, is set free. The office of the sulphuric acid is to dissolve 
the oxide of the metal as soon as produced, and form with it a salt (sulphate 
of zinc), thus inducing a decomposition of the water. 



860 SOLAR MICROSCOPE. 

opened, and the gas allowed to blow oif ; this should be done 
twice^ so as to allow the bell to be freed from any mixture of 
atmospheric air before a flame is applied ; otherwise an explo- 
sion,' caused by the impurity of the hydrogen, may result upon 
the first application of a flame to the jet. 

The generator, thus prepared, may be kept for use at several 
exhibitions, or until the action of the liquid has become too 
feeble, owing to the union of the acid with the dissolved metal, 
to form a sulphate of zinc, when a fresh mixture should be 
substituted. If the hydrogen is to be received in a bag, after 
passing the purifier, as seen in Fig. 273, this bag should be 
freed from air, and attached as in the case of oxygen already 
described. 

To regulate the flame of the oxhydrogenjet^ Fig. 271. 
— With the stop-cocks attached to the bag and generator 
entirely open, turn the small stop-cock of the tube connect- 
ing with the latter, and allow nearly a full flow of hydro- 
gen; ignite this with a taper, and then let on, through the 
other small stop-cock, oxygen, until the jet is reduced to a small 
bluish- white flame; such a flame causes intense heat, which 
will soon cause the lime to become of a most brilliant white. 
A very good light may be obtained by substituting for the hydro- 
gen bi-carburetted hydrogen (street gas), or even by allowing 
only a jet of oxygen to blow through the flame of a lamp, upon 
a piece of lime properly arranged. 

The Solar Microscope. — This is similar in its operation to 
the oxhydrogen microscope just described. For illuminating 
the object in this, solar light is employed. The tube contain- 
ing the lenses is fixed in an opening of a window-shutter, and 
a mirror is placed upon the outside in such a position as to re- 
flect the sun's light upon the condensing lens. No artificial 
light can equal this for illumination ; but, as the solar microsco})o 
can be used only by day, and when the sky is unclouded, it is 
less serviceable for popular exhibitions than the oxhydi'ogen 
microscope, which is best used at evening. 



BENZOLE LIGHT. 



3G1 



The Benzole Light, — An interesting application of hy- 
drogen has, within a few years, been made to the production 
of the celebrated Benzole or Water Light. By means of 
a variety of ingeniously-contrived machines, this light, which 




considerably exceeds in brilliancy the common gas light, 
has been applied to purposes of illumination, and many 
large factories, and public as well as private dwellings, are 
now lighted from this source. 

Fig. 277 presents a simple apparatus for exhibiting this 
31 



362 BENZOLE LIGHT. 

light in the lecture-room, or for individual amusement. A, 
B, and Z, represent the acidulated water, the copper bell, 
and roll or bucket of zinc, of the hydrogen generator, as 
already described. From the stop-cock of this leads the rub- 
ber hose, T, for conveying the hydrogen gas to the water 
of the purifier, P ; this hose is slipped upon the end of the 
right-angular glass tube, which enters the water, W, after 
which, it is conveyed through the second rubber hose, C, down 
into a small quantity of benzole contained in the second 
bottle, at D. From this the hydrogen receives a due propor- 
tion of carbon, the luminous principle of flame, and the com- 
pound gas thus formed (carburetted hydrogen) escapes through 
a small jet at L, where it is burned, as shown in the figure. 
S, S, are safety-tubes, up which the liquids may rise when- 
ever there is an undue pressure. In this way the flow of 
gas from the jet is made uniform, and any flickering of the 
flame prevented. 

The benzole employed for this light is a volatile oil, re- 
sembling naphtha, which contains a very large proportion of 
carbon, and which it readily yields to the hydrogen when 
passed through it. Common air, slightly warmed and passed 
through this oil, will yield a very brilliant light, although 
inferior to that from pure hydrogen. The glass tube should 
enter the benzole only about half an inch, lest the hydrogen 
become too much carbonized, so as to cause the flame to 
smoke. 

The directions for filling the hydrogen generator have 
been already given. We repeat, that, when filled, the gas 
formed in the bell should be drawn off* at least twice, and 
the stop-cock, after each time, immediately closed. Any mix- 
ture of common air with hydrogen forms an explosive com- 
pound. Where the benzole light is employed for illum- 
inating dwellings, a machine is usually placed in the cellar, 
and the generation of the illuminating vapor regulated by 
a crank above, which is occasionally turned. With proper 



BENZOLE LIGHT. 363 

skill in using, this benzole light forms both a cheap and bril- 
liant illumination. 

The expense of a light procured from the passage of atmos- 
pheric air through this liquid is much less than from many of 
the ordinary methods of illumination out of large cities. The 
chief obstacle in the way of obtaining light from this source is 
the readiness with which this hydro-carbon vapor condenses ; 
thus, in cold weather, this becomes condensed in the tubes which 
pass through rooms which are not properly warmed, so as soon 
to clog these and prevent the flow of the gases. 



INDEX. 



A. 

Animal Electricity, 241. 
Air, Atmospheric, 87. 
" type of liaids, obvious properties, 

87. 
«« materiality shown by its visibility ; 

inertia, 8vt. 
" resistance of; guinea and feather 

experiment, 90. 
*« fall of liquids in vacuo, 91. 
*< buoyancy of, 92. 
« impenetrability of, 93. 
*' its weight and pressure, 94, 104. 
" to weigh, 96. 

" condensed; effect on animal sys- 
tem, 100. 
*« upward pressure shown, 106. 
« fluidity shown, 112. 
" elasticity of ; Mariotte's law, 113. 
** compressed; elastic force of, 118. 
** conductor of sound, 142. ^ 
" its conducting power varies, 143. 
Air-pump described, 77. 

<* theory of its operation, 79. 

«* directions for its use, 80. 

Attraction, two kinds defined, 16. 
Archimedes' screw, 73. 
Atwood's machine, 27. 
Action and reaction illustrated, 31. 
Abbe Nollet's globe, 193. 
Aurora illustrated, 194. 
Aurora Borealis, 214. 

B. 

Bodies, solid, fluid and gaseous, 16. 
** self-luminous, 286. 
** opaque, transparent, translu- 
cent, 287. 
«* falling, laws of, 26. 



Balance and steel-yard, 37. 
Barker's mill, 72. 
Bell, diving, illustrated, 93. 
Bolt-head, 103. 
Barometer and its uses, 107. 
Balloon, pneumatic, 114. 
Bacchus experiment, 116. 
Balance electrometer, 182. 
Benzole light, 361. 
Beer-jar, 120. 

Battery, galvanic, compound, 218, 222. 
*« thermo-electric, 240. 

C. 

Centre of gravity, to find this, 20. 

< " falls before point of 

support, 21. 

' «* illustrations of, 24. 

Capstan, 41. 
Condenser described, 82. 
Condensing-chamber, 119. 
Clouds, 137. 
Calorific rays, 311. 
Cold, causes of, 341. 

produced by evaporation, 342. 

D. 

Directing-rod, 171. 

Decomposition by electric spark, 200. 

" by galvanism, 227. 

Daguerreotype, 313. 
Dissolving views, arrangement of the 

lanterns, 350. 
Drummond light, preparation of oxy- 
gen for this, 355. 
<« " preparation of oxy- 

gen from manga- 
nese, 357. 
Dew, how formed, 138. 



INDEX. 



Equilibrium, stable, \instable and neu- 
tral, 22. 
Engine, fire, explained, 122. 
" atmosphei-ic, 128. 
*' Watt's improved steam, 130. 
•* high-pressure, 132. 
Eolopile, 127. 
Ear, how produces hearing, 152. 

" of Dionysius, 147. 
Echo, 146. 

Electricity, mechanical^ introduction 
to, 165. 
" history of; theories of, 165. 

** produced by friction of 

glass, 174. 
" produced by cloths, steam, 

175. 
" two kinds, 176. 

** induction of, theory, 184. 

** resides on surface of the 

glass jar, 189. 
** illumination by, different 

colors, 190. 
* * passage of through rarefied 

air, 193. 
** combustion by, 195. 

** mechanical effects of, 198. 

** from points, 201. 

** agency in evaporation, 

204. 
" effects on the animal sys- 

tem, 204. 
" virtues of as a medical 

agent, 205. 
'* of the atmosphere, 207. 

«* of thunder-clouds, 209. 

** return-stroke, 210. 

** galvanic, history of its 

discovery, 216. 
** manner produced, 217. 

" difference between it and 

mechanical, 219. 
** heating effects, 233. 

** effect on the animal sys- 

tem, 236. 
'* f/ierrno, how produced,239. 

" animal, 241. 

" magneto, 283. 

Electric machine described, 167. 
" theory of, 177. 
" battery, 171. 
" attraction and repulsion, 178. 
" sportsman and birds, 179. l 
" dancing images ; spider ; j 
swing-bells, 180. 



Electric swan, 182. 

** induction shown by double 

jar, 188. 
" luminous frame; star, 191. 
" luminous tubes; jar, 192. 
** cannon, manner of filling, 

197. 
*' spark, inflammation of ether 

by, 198. 
*' mortar, 201. 
** currents, attraction of, 278. 
'* secondary curi'ents, 278. 
'* helix, polarity of, 251. 
Electro-magnetism; effects of a flow of 
electricity in caus- 
ing magnetism, 
245. 
** " theory of earth'3 

magnetism, 255. 
" machines for revolving by, 272. 
" metallurgy, 231. 
Eye, its structure, 315. 
" images formed on its retina, 316. 
" power of adapting itself to dis- 
tances, 317. 
" how magnitude of distant objects 

determined, 318. 
" near and far sightedness, 819. 
" impressions on its retina, 320. 
" inability to distinguish colors, 
322. 
Electrometer, gold-leaf, 173. 
pith-baU, 172. 
Electrophorous, 185. 



Forces, centrifugal and centripetal, 

29. 
Fly-wheel, 49. 
Fusee in watch-work, 51. 
Friction, 55. 
Fountain in vacuo, 117. 
Freezing mixtures, 846. 
Floating bodies, 69. 
Fishes, sink and rise, 115. 

G. 

Gravitation, tendency to draw bodies 
" to the earth's centre, 

18. 
*♦ acts alike on all bodies, 

19. 
Gravity, centre of, to find this, 20. 
* * " falls before the point 

of support, 21. 



367 



Gravity, illustrations of, 24. 

" specific, illustrated, 66. 

" to ascertain that of solids, 67. 
" " liquids, 68. 

Guinea and feather tube, 84. 
Gymnotus, 241. 
Galvanometer, 246. 

H. 

Hydrostatic press, to ascertain pressure 
of this, 58. 
*' bellows, 61. 

" paradox, 63. 

" balloon, 114. 

Hydrometer, 68. 
Hydraulics defined, 70. 
Hillotype, 313. 
Heat, sources of, 333. 
" expansion of solids by, 336. 
" equilibrium of, 337. 
*' conductors, metals good, 337. 
** liquids; bad conductors of ; how 

heated, 339. 
*' radiation of, 339. 
*' reflection, 340. 
*' transmission of, 341. 
Hand-glass, 96. 

Hydrogen generator, to prepare this, 
359. 

I. 

Inclined plane, 43. 

K. 

Kaleidoscope, 294. 



Lever, three kinds illustrated, 86. 

" compound, 39. 
Liquids, flow of; resistance of, 71. 
" pressure of, 57. 
" rise of in tubes, 95. 
" boiling of under pressure, 110. 
" equilibrium of, 60. 
** conduct sound, 145. 
*' pressure of illustrated, 63. 
Leyden jar, theory of, 186. 
Lightning-rods, 211. 

" safety from lightning, 

213. 
Light, two theories of; rate of progress, 
286. 
•* course of; shadow and penum- 
bra, 288. 



Light, intensity of; diminishes with 

distance, 289. 
" reflection of, 291. 
•' refraction, of, 298. 
** limiting angle of refraction, 

300. 
** refraction of by double-convex 

lenses, 302. 
*' refraction of by double-concave 



" decomposition of, 305. 
** polarization of, 310. 
" chemical action of, 311. 
Lenses, 301. 

'* images formed by, 304. 
•* achromatic, 307. 

M. 
Matter, properties of; extension; im 
penetrability; divisibility; 
figure; porosity, 13. 
" inertia of, 15. 
" specific properties of, 17. 
Momentum, 26. 

Motion, ditterent kinds defined, 25. 
*' of projectiles, 28. 
*' central, how produced, 29. 
" reflected, 32, 
" resultant, 32. 
Mechanical powers; machines, how 

composed, 36. 
Machinery, 47. 

' ' relation between power and 

resistance in, 48. 
Magdeburgh cups, pressure on these, 

99. 
Marcet's steam globe, 127. 
Magnetism, 154. 

Magnets, natural and artificial, 154. 
** how arrange themselves 
when free to move, 154. 
Magnetic induction, theory of, 156. 
Magnetism of soft iron; of steel, 157. 

'* terrestrial, 158. 

Magnets, artificial, how made, 157. 
*' reaction of; their revolution, 

248. 
" electro, how made, 256. 
Magnetizing helices, 257. 
Mirage, 301. 
Microscope, simple, 322. 
" compound, 324. 

*« solar, 360. 

" advantages of, 326. 

*• oxhydrogen, how arrang- 

ed, 353. 



368 



INDEX. 



Magic lantern, how constructed, 346. 
*« directions for use of, 

348. 
Mercury, height atmosphere will sus- 
tain this, 104. 
Meteorology, 135. 
Mists, 137. 
Muscular action, theory of, 243. 

N. 

Needle, magnetic, 159. 
** its declination, 160. 
" its dip, 161. 
" astatic, 247. 

0. 

Optical instruments, 322. 
Oxhydrogen jet, to regulate the flame 
of, 360. 

P. 

Pulleys, two kinds, 42. 
Pendulum described, 51. 

•* laws of vibration of, 52. 

** used for determining the 
figure of the earth, 53. 

** As a measure of time, 54. 
Pneumatic instruments, description of 
76. 

*• paradox, 120. 
Pump, lifting, 121. 
Photogi'aph, 312. 
Phantasmascope, 321. 
Pyrometer, 336. 
Phantasmagoria, 350. 

R. 

Rain; hail, 138. 
Reflectors, curved, 295. 

*' images from these, 296. 

** convex, images formed by, 

297. 
Rainbow, 308. 



Suction, absurdity of, 105. 
Siphon, 123. 



Sound, how caused, 141. 

" solids conduct this, 144. 

•' progressive, 145. 

** reflection of, 146. 

" musical, 148. 

*' theory of musical sounds, 149. 
Signal-key, 265. 
Shocking-machines, 279. 
Screen, 350. 

Stop-cock, dissolving, 351. 
Screw connections, 85. 

T. 

Telegraph, electro-magnetic, Morse's, 
260. 
** ** House's print- 

ing, 266. 
*« " Fire alarm, 271. 

" Hughes', 266. 
Telescope, reflecting, 327. 
" refracting, 328. 

<' terrestrial, or spy-glass, 330. 
«' GaUleo's, 332. 
Thermometer, air, 334. 

** mercurial, 335. 



Universal discharger, 195. 

V. 

Vapor, expansion and elastic force of, 

126. 
Voice, 151. 
Ventriloquism, 151. 

W. 

Water frozen by its own evaporation 
in vacuo, 343. 
" as a motive power ; water- 
wheels, 72. 
" sustained in an inverted jar, 
103. 
Wood, porosity of shown^ by atmos- 
pheric pressure, 102. 
Winds, land and sea breezes, 135. 

" trade-winds, 137. 
Whirlwinds, 136. 



SARGENT'S 



STA^fDARD 




SERIES 



DEI. JE3 .^^ 13 IHS IFL @» . 



NEW AND IMPROVED SEKIES. 
By EPES SARGENT, 

AUTHOR OP "THE 8TANDABD SPEAKER." 



5. The First-Class Standard Reader. 

4. The Standard Fourth Reader. 

3. The Standard Third Reader, or Guide to Articulation. 

2. The Standard Second Reader. With illustrations by Billings. 

1. The Standard Primer, or First Reader, with lUustrations 

by Billings. 

Mr. Sargent has been several years engaged in the preparation of these 
works. Besides every well-tested improvement, they embrace some novel 
features that give the series peculiar claims upon the favor of teachers. 

Great attention is paid, especially in the more elementary books, to the 
subject of articulation and pronunciation. 

The First-Class Standard Reader, and the Standard Fourth Reader. 
are now ready, and have called forth the most decided commendations. 

The First-Class Standard Reader has passed through ten lai-ge 
editions in less than six months from its appearance in the market — a suc- 
cess quite unparalleled in the history of American school-books. It forms 
a beautiful H!mo of 478 pages. 

The Standard Fourth Reader is meeting with a success quite equal to 
that which has attended its predecessor. It is pronounced the handsomest 
Reader y mechanically considered, in the market ; while its intrinsic superiority is 
not less apparent to a critical examination. It contains a thorough course 
of preliminary exorcises in Articulation, Pronunciation, Accent, &c.\ nu- 
merous Exercises in Reading ; a new system of References, and a copious 
Explanatory Index. The reading exercises have been graduated with ex- 
treme care to the tastes and capacities of the class of pupils next to that 
for which the First-Class Reader is designed. The Standard Fourth 
Reader forms a beautiful 12mo of 336 pages, long primer type, leaded. 



The Standard Third Reader will be ready in March, 1855 ; and the 
concluding volumes of the series will rapidly follow. 

^^* Copies of these Readers, for examination, will be supplied gratu- 
itously to teachers, subject to their orders as to conveyance. By sending 
24 cents in postage stamps for the First-Class Standard Reader, or 18 
cents in stamps for the Standard Fourth Reader, they can be sent by 
mail to any part of the United States, postage prepaid. 

PHILLIPS, SAMPSON & Co., Publishers, 

13 WiKTER St. Boston. 



PHILLIPS, SAMPSON, & CO.'S PUBLICATIONS. 



SARGENT'S SIX CHARTS, (23 inclies by 30,) 

For use in Teaching, Reading, Spelling, &c., in Primary 

Schools. 

Price, $1.25. 

An idea of the distinguishing characteristics of the two highest Readers of this 
Bories may be got from the following passage from the Annual Report of the 
school committee of the city of Lynn, Mass., for 1854. 

"Only one change has been made by the committee in the text-books used in 
the schools. This was the introduction of Sargent's Standard Fifth Header, 
and his Standard Fourth Reader into the gi-aniinar schools. Some change was 
greatly needed. The teachers were almost unanimous in the condemnation of 
the reading book.s which were used in the schools, and in this condemnation the 
committee, after a patient and full examination, entirely concurred. 

"The two reading books introduced, it is believed, will meet admirably the 
wants of the schools. The selection of pieces is made with tact and excellent 
judgment, exhibiting a rare acquaintance with the best sources of our literature, 
as well as a nice perception of the wants of teacliors and pupils in the exercises 
reqiiisite to form a good taste and a correct style in reading. 

"The directions given for prouunciution-, iullcction, and articulation are also 
very full, care being taken to point out the most common faults and errors. 
These merits, together with the copious references to the best authorities, and the 
explanatory indexes, render these books truly standard reading books. If tho 
remaining books of the series equal these, and are as well adapted to the inter- 
mediate and primary, as these to the grammar schools, they will supply a need 
which has long existed of a complete and uniform series of reading books for all 
the schools." 

From an active friend of the cause of education in Illinois, George M. Dewey, 
Esq., of Antioch, we have the following testimonial : — 

" This series of books I believe superior to any others now in use. Pirst, he- 
cause the elemental nidiments of the language are there better explained than I 
have before seen them. Secondly, because the rules therein laid down for the 
government of the voice are just what the interest of the scholar requires — 
brief, concise, and to tlie point. Thhdly, because the selections composing the 
reading exercises are among the finest specimens of the literary productions to he 
found in the language — selected with great care by one of the finest scholars of 
the age. And, fiinally, because there is attached to it an explanatory index, in 
which may be found the definition and derivation of all words of difficult orthog- 
raphy or pronunciation ; also, the history in brief of each author ft-om whose 
writings selections have l)eeu made for the vohnnes. This feature I believe en- 
tirely new, and surely it is one that commends itsi-lf to every scholar and teacher 
as an improvement long called for by the interests of education. 

"No person at all conversant with tlie wants of our schools %vill hesitate in 
saying that the Readers now used are not what the necessities of the rising gen- 
eration require, and that a radical change is demanded, provided a superior hook 
can be obtained. Such a series I believe those of Mr. Sargent's to be, in every 
particular. I think them as much above those now in use as they are above the 
old English Reader of by-gone years." 



THE PRINCIPLES OF CHEMISTRY, 

Illustrated by simple Experiments. By Dr. Julius Adolph 
Stockhardt, Professor in the Royal Academy of Agriculture at 



PITTLLTPS, SAMPSON. & CO.'S PUBLICATIONS. 

Tharaud, and Royal Inspector of Medicine in Saxony. Trans- 
lated by C. H. Pierce, M. D., with an Introduction by Professor 
E. N. ilorsford, of Cambridge. Price, $1.75. 

Extract from a Letter of S. L. Dana, M. D., LL. D. 

This book is preeminently clear, concise, practical in all its allusions to art, 

simple in its arranjxcmcnts, and illustrated by experiments requiring no array of 

costly apparatus. It is a wnrlc worthy of its author, and will bear the character 

we have given to it, even when subjected to the severest scrutiny. 

From A. A. Hayes, M. D., Assayer to the State of Massachusetts. 

After reading this work in the translation by Dr. Tierce, I have formed the 
opinion that, as an easy introduction of the student to the i)rinciples of chemis- 
try, it is unrivalled by any book in our language, llaroly is it possible to find an 
elementary work which, without being voluminous, discusses so many subjects 
clearly. 

Extract from Professor Horsfordi's Introduction. 

The qualifications of this work as a text book for p; hools are such as to leave 
little, if any thing, to be desired. The classification is exceedingly ' convenient. 
The elucidations of iiriiicijiles and tho explanations of chemical phenomena are 
admirably cli'ar and concise. TIk^ book is also well adapted to the wants of teach- 
ers who (losing to give occasional experimental lectra-es at a moderate expense, 
and of those who design to commence the study of chemistry, either with or 
without the aid of an instructor. 

From John A. Porter, Professor of Chemistry applied to Art, in Tale College. 

I concur entirely in the views of the work expressed by Professor Horsford in 
the introduction, and shall recommend it to those pursuing the study of chemis- 
try under my direction. 

From David A. Wells, Practical Chemist. 
I consider Stockhardt's " Principles of Chemistry," as an elementary book, 
superior to any other woiic of the kind hitherto published. 

I have carefully studied Stockhardt's Chemistry, and have used it in the in- 
Btruction of my classes. As a text book it is worthy of all praise; far better than 
any I have examined since the progress of science rendered the Conversations on 
Chemistry obsolete. The original is the work of a man at once skUful as a 
teacher, and profound in his knowledge of the history and principles of his sci- 
ence, and familiar %\ith facts and the details of maniiiulation. The translation is 
faultless. It has entirely the air of an original ; and in simpUcity, clearness, and 
conciseness may be regarded as a model. 

GEORGE B. EMERSON. 



Muglanh 



INTELLECTUAL PHILOSOPHY, 

By the author of "Elements of Moral Science," "Political 
Economy," &c., President of BroAVTi University. This work is 
designed for Colleges, Academies, and High Schools. $1.25. 
"President "Wayland's 'Elements of Intellectual Philosophy' will supersede 
other treatises on the subject, as a text book for students. His work has peculiar 
fitness for that position. It is the gradual growth of a series of lectures to suc- 
cessive classes in Brown Universit3'. Composed with this view, it is admirably 
Bdapted to those who are beginning the study of mental science." — Boston Post. 

"The reputation of Dr. Wayland as a healthy and deep thinker is established 
upon so firm a basis, that a new text book from his pen will be sure to meet with 
an extensive demand for many years." — Boston Transcript. 



PnTLLTPS, SAMPSON, & CO.'S PUCLTCATIOXS. 

" We know of no work so well adapted to popularize intellectual philosophy as 
this. We venture to predict that it will become the most popular text book on 
the subject iu this country." — Zion's Herald. 

" We commend it to the attention of Professors who have charj^e of classes iu 
nient.al science in our collo;i;es. From a cursorv examination, we think it de- 
cidedly superior to the author's work on Moral Philosophy, which is extensively 
known to the public." — Christian Observer. 

" It is the better adapted for a text book in High Schools and Colleges, that the 
form in which the lectures were prepared for oral delivery is retained, with which 
perhaps might seem a redundance of illustration, if experience had not estab- 
lished the necessity of it in order to fix definitely iu the mind of the pupil the 
nature and limits of subjective truth." — JSew York Journal of Commerce. 

*< In every page of this work is to be seen President Wayland's accustomed 
clearness and compactness of style, felicity of illustration, and force of argument; 
the result of frequent revision aiid loug and careful preparation." — National Era. 

" The work of Dr. Wayland will fill a long vacant place in the department of 
intellectual science. There has not before been any work well fitted for use as a 
text book in this branch of study in our High Schools and Colleges. It was 
needed and will be exteudively used." — Korton's Gazette. 

"This work imbodies the ripe fruits of a lifetime. Its arrangement, condensa- 
tion, and perspicuity are every way adinirublo. All that precision, clearness, 
simplicity, and force of language can do to make the most perplexing of all sci- 
ences plain, is here set forth. In the reading of these chapters we are reminded 
of the saying of the great and good Archbishop Leighton to his clergy : ' How 
much learning it takes to make these things plain 1 ' " — National Intelligencer. 



ESSAYS m THE INTELLECTUAL POWERS OF MAN, 

By Thomas Reid, D. D., F. E. S. E. Abridged, with Notes 
and Ilhistrations from Sir William Hamilton and others. Ed- 

• ited by James Walker, D. D., President of Harvard College. 
One volume, 12mo. Price, $1.25. 
The works of Keid and Stewart are too widely known to need comment, and 

whatever other treatises may be pubUshed, these will probably always retain their 

admirei's. 



TEE PEILOSOPnY OF THE ACTIVE AND MORAL POWERS 
OF MAN, 

By Dugald Ste^vart, F. 11. 8., London and Edinburgh. Re- 
vised by James Walker, 1). D., President of Harvard College. 
One volume, 12mo. Price, 1.2o. 

5* 



^^c. 



> 













^ /\ 



6"' .^ 






^ "^6^ : 


















.?!^^^.^SEk^.f^^ 







